runtime: reduce pthread stack size in TestCgoCallbackGC
runtime: reduce pthread stack size in TestCgoCallbackGC

Fixes #11959

This test runs 100 concurrent callbacks from C to Go consuming 100
operating system threads, which at 8mb a piece (the default on linux/arm)
would reserve over 800mb of address space. This would frequently
cause the test to fail on platforms with ~1gb of ram, such as the
raspberry pi.

This change reduces the thread stack allocation to 256kb, a number picked
at random, but at 1/32th the previous size, should allow the test to
pass successfully on all platforms.

Change-Id: I8b8bbab30ea7b2972b3269a6ff91e6fe5bc717af
Reviewed-on: https://go-review.googlesource.com/13731
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Martin Capitanio <858ab70fbaf935929af08272b5559debcd7281dd@gmail.com>
Reviewed-by: Minux Ma <72d850dbd8858b791995463d8539b5a379577f28@golang.org>

theass/go,cyberroadie/go,goodlang/good,vanloswang/go,rachtsingh/gogojuice,samanalysis/go,kim-racktop/go,Endika/go,mwhudson/go,nawawi/go,likesea/go,nawawi/go,stormltf/go,suncycheng/go,dterei/go,rachtsingh/gogojuice,cloudflare/go,dpatel06/go,akutz/go,ichu501/go,xiuzhifu/go,dterei/go,yasarkunduz/go,rachtsingh/gogojuice,kim-racktop/go,golang/go,cookerzhu/go,miolini/go,theass/go,josharian/go,NunoEdgarGub1/go,ichu501/go,cookerzhu/go,odeke-em/go,jdhenke/go,kim-racktop/go,stormltf/go,jacobhaven/go,josharian/go,b54898533/go,cloudflare/go,jonasi/go,yasarkunduz/go,xiuzhifu/go,NunoEdgarGub1/go,jacobsa/go,cyberroadie/go,suncycheng/go,goodlang/good,NunoEdgarGub1/go,stormltf/go,theif519/go,Badredapple/go,CarterTsai/go,beealone/go,ichu501/go,xiuzhifu/go,vsekhar/elastic-go,vanloswang/go,devpadawan/go,codestation/go,4ad/go,snowsnail/go,nawawi/go,malvira/go,ichu501/go,skatsuta/go,vsekhar/elastic-go,cloudflare/go,beealone/go,xiuzhifu/go,likesea/go,CarterTsai/go,NunoEdgarGub1/go,CAFxX/go,momchil-velikov/go,sndnvaps/go,ichu501/go,joshsammut/go,Endika/go,theass/go,ImJasonH/go,goodlang/good,malvira/go,polyverse-security/go,codestation/go,danielsoro/go,gdey/go,Jordanzuo/go,ganboing/go-esx,nawawi/go,mwhudson/go,Jordanzuo/go,vsekhar/elastic-go,momchil-velikov/go,jacobhaven/go,b54898533/go,miolini/go,gdey/go,roth1002/go,dpatel06/go,4ad/go,Endika/go,Pulgafree/go,rachtsingh/gogojuice,vanloswang/go,goodlang/good,Cofyc/go,mwhudson/go,likesea/go,Cofyc/go,ericsnowcurrently/go,ganboing/go-esx,jdhenke/go,samanalysis/go,b54898533/go,kim-racktop/go,danielsoro/go,ifagnosticism/go,jacobhaven/go,jonasi/go,suncycheng/go,Jordanzuo/go,theif519/go,linux-on-ibm-z/go,vsekhar/elastic-go,zhangg/go,polyverse-security/go,samanalysis/go,yasarkunduz/go,suncycheng/go,derekmarcotte/go,ImJasonH/go,alex-zhang/go,ImJasonH/go,danielsoro/go,myPublicGit/go,CarterTsai/go,kim-racktop/go,golang/go,b54898533/go,joshsammut/go,miolini/go,odeke-em/go,cyberroadie/go,CarterTsai/go,jdhenke/go,jacobsa/go,jacobsa/go,GeorgiCodes/go,snowsnail/go,goodlang/good,dpatel06/go,jacobhaven/go,samanalysis/go,jacobsa/go,nawawi/go,deft-code/go,stormltf/go,jonasi/go,yasarkunduz/go,fjballest/golang,Pulgafree/go,miolini/go,devpadawan/go,codestation/go,theass/go,theif519/go,yasarkunduz/go,Jordanzuo/go,CAFxX/go,stormltf/go,gdey/go,codestation/go,codestation/go,GeorgiCodes/go,zhangg/go,roth1002/go,Samurais/go,snowsnail/go,jonasi/go,mwhudson/go,mwhudson/go,likesea/go,yasarkunduz/go,skatsuta/go,snowsnail/go,ImJasonH/go,sndnvaps/go,Cofyc/go,ganboing/go-esx,polyverse-security/go,xiuzhifu/go,ichu501/go,theif519/go,linux-on-ibm-z/go,goodlang/good,polyverse-security/go,kim-racktop/go,suncycheng/go,deft-code/go,jonasi/go,alex-zhang/go,mikekap/go,b54898533/go,golang/go,Badredapple/go,dpatel06/go,binarycrusader/go,sndnvaps/go,Badredapple/go,CarterTsai/go,ichu501/go,theass/go,snowsnail/go,odeke-em/go,Jordanzuo/go,ifagnosticism/go,ifagnosticism/go,jacobsa/go,alex-zhang/go,deft-code/go,Cofyc/go,vanloswang/go,gdey/go,ifagnosticism/go,codestation/go,zhangg/go,mikekap/go,danielsoro/go,likesea/go,Pulgafree/go,mikekap/go,vanloswang/go,derekmarcotte/go,akutz/go,Badredapple/go,momchil-velikov/go,4ad/go,ifagnosticism/go,Jordanzuo/go,devpadawan/go,beealone/go,joshsammut/go,kim-racktop/go,akutz/go,ifagnosticism/go,CAFxX/go,ImJasonH/go,ganboing/go-esx,momchil-velikov/go,cyberroadie/go,roth1002/go,suncycheng/go,joshsammut/go,josharian/go,GeorgiCodes/go,cloudflare/go,vanloswang/go,miolini/go,ericsnowcurrently/go,deft-code/go,odeke-em/go,roth1002/go,vsekhar/elastic-go,linux-on-ibm-z/go,malvira/go,cyberroadie/go,golang/go,myPublicGit/go,Samurais/go,jdhenke/go,ImJasonH/go,dterei/go,mikekap/go,joshsammut/go,Pulgafree/go,devpadawan/go,Endika/go,sndnvaps/go,beealone/go,derekmarcotte/go,vanloswang/go,goodlang/good,zhangg/go,Jordanzuo/go,4ad/go,josharian/go,alex-zhang/go,theif519/go,kim-racktop/go,joshsammut/go,samanalysis/go,jdhenke/go,malvira/go,b54898533/go,danielsoro/go,deft-code/go,vsekhar/elastic-go,rachtsingh/gogojuice,ericsnowcurrently/go,golang/go,skatsuta/go,ganboing/go-esx,Samurais/go,linux-on-ibm-z/go,ifagnosticism/go,danielsoro/go,cookerzhu/go,4ad/go,Badredapple/go,myPublicGit/go,polyverse-security/go,devpadawan/go,josharian/go,malvira/go,b54898533/go,skatsuta/go,golang/go,beealone/go,rachtsingh/gogojuice,fjballest/golang,GeorgiCodes/go,gdey/go,danielsoro/go,dterei/go,odeke-em/go,beealone/go,Pulgafree/go,codestation/go,josharian/go,deft-code/go,Cofyc/go,linux-on-ibm-z/go,linux-on-ibm-z/go,myPublicGit/go,golang/go,Samurais/go,miolini/go,zhangg/go,dterei/go,xiuzhifu/go,alex-zhang/go,miolini/go,jacobhaven/go,skatsuta/go,binarycrusader/go,devpadawan/go,snowsnail/go,Samurais/go,mikekap/go,codestation/go,linux-on-ibm-z/go,polyverse-security/go,cookerzhu/go,polyverse-security/go,b54898533/go,goodlang/good,mwhudson/go,fjballest/golang,fjballest/golang,GeorgiCodes/go,jdhenke/go,ganboing/go-esx,dterei/go,Samurais/go,alex-zhang/go,roth1002/go,mikekap/go,vsekhar/elastic-go,snowsnail/go,nawawi/go,jacobsa/go,jonasi/go,NunoEdgarGub1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