misc/cgo/testsanitizers: fix check for existence of clang
misc/cgo/testsanitizers: fix check for existence of clang

The current mechanism fails if clang cannot be executed by the current
user.  Using the `-x` operator for `test` return TRUE if the file is
executable by the user.

Change-Id: I0f3c8dc3880c1ce5a8a833ff3109eb96853184af
Reviewed-on: https://go-review.googlesource.com/16752
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

deft-code/go,ganboing/go-esx,Samurais/go,Cofyc/go,vsekhar/elastic-go,nawawi/go,mwhudson/go,vanloswang/go,deft-code/go,Samurais/go,vanloswang/go,kim-racktop/go,odeke-em/go,golang/go,vsekhar/elastic-go,josharian/go,fjballest/golang,suncycheng/go,akutz/go,binarycrusader/go,derekmarcotte/go,mwhudson/go,cloudflare/go,roth1002/go,fjballest/golang,suncycheng/go,rachtsingh/gogojuice,vanloswang/go,ganboing/go-esx,fjballest/golang,ImJasonH/go,derekmarcotte/go,goodlang/good,akutz/go,CAFxX/go,roth1002/go,ImJasonH/go,theif519/go,Endika/go,ganboing/go-esx,Samurais/go,miolini/go,roth1002/go,roth1002/go,binarycrusader/go,cyberroadie/go,miolini/go,linux-on-ibm-z/go,golang/go,4ad/go,codestation/go,fjballest/golang,cloudflare/go,theif519/go,derekmarcotte/go,roth1002/go,miolini/go,nawawi/go,josharian/go,Cofyc/go,akutz/go,sndnvaps/go,polyverse-security/go,polyverse-security/go,deft-code/go,deft-code/go,4ad/go,Cofyc/go,momchil-velikov/go,cloudflare/go,vsekhar/elastic-go,theif519/go,momchil-velikov/go,Endika/go,derekmarcotte/go,kim-racktop/go,nawawi/go,theif519/go,linux-on-ibm-z/go,cyberroadie/go,linux-on-ibm-z/go,codestation/go,mwhudson/go,CAFxX/go,sndnvaps/go,fjballest/golang,Samurais/go,akutz/go,Jordanzuo/go,sndnvaps/go,CAFxX/go,sndnvaps/go,CAFxX/go,josharian/go,linux-on-ibm-z/go,Samurais/go,binarycrusader/go,kim-racktop/go,cyberroadie/go,odeke-em/go,cyberroadie/go,Endika/go,miolini/go,polyverse-security/go,goodlang/good,vsekhar/elastic-go,suncycheng/go,theif519/go,theif519/go,derekmarcotte/go,Samurais/go,sndnvaps/go,suncycheng/go,vsekhar/elastic-go,Samurais/go,cloudflare/go,goodlang/good,Endika/go,mwhudson/go,Jordanzuo/go,codestation/go,rachtsingh/gogojuice,suncycheng/go,codestation/go,CAFxX/go,mwhudson/go,kim-racktop/go,ImJasonH/go,momchil-velikov/go,cyberroadie/go,theif519/go,goodlang/good,CAFxX/go,Cofyc/go,cyberroadie/go,golang/go,ganboing/go-esx,ImJasonH/go,CAFxX/go,odeke-em/go,derekmarcotte/go,cloudflare/go,momchil-velikov/go,Jordanzuo/go,linux-on-ibm-z/go,Cofyc/go,fjballest/golang,derekmarcotte/go,deft-code/go,codestation/go,vanloswang/go,odeke-em/go,goodlang/good,josharian/go,binarycrusader/go,nawawi/go,nawawi/go,momchil-velikov/go,goodlang/good,Endika/go,fjballest/golang,fjballest/golang,Jordanzuo/go,ImJasonH/go,odeke-em/go,derekmarcotte/go,roth1002/go,4ad/go,4ad/go,Samurais/go,4ad/go,golang/go,codestation/go,deft-code/go,ganboing/go-esx,vsekhar/elastic-go,cloudflare/go,polyverse-security/go,josharian/go,golang/go,cyberroadie/go,rachtsingh/gogojuice,sndnvaps/go,linux-on-ibm-z/go,suncycheng/go,Endika/go,ImJasonH/go,momchil-velikov/go,polyverse-security/go,mwhudson/go,Endika/go,deft-code/go,mwhudson/go,nawawi/go,josharian/go,codestation/go,ganboing/go-esx,roth1002/go,roth1002/go,ImJasonH/go,linux-on-ibm-z/go,vsekhar/elastic-go,golang/go,sndnvaps/go,rachtsingh/gogojuice,polyverse-security/go,codestation/go,mwhudson/go,suncycheng/go,miolini/go,cloudflare/go,josharian/go,kim-racktop/go,odeke-em/go,kim-racktop/go,odeke-em/go,binarycrusader/go,akutz/go,nawawi/go,rachtsingh/gogojuice,sndnvaps/go,Jordanzuo/go,vsekhar/elastic-go,akutz/go,4ad/go,vanloswang/go,deft-code/go,rachtsingh/gogojuice,christopher-henderson/Go,theif519/go,Cofyc/go,nawawi/go,goodlang/good,akutz/go,binarycrusader/go,miolini/go,rachtsingh/gogojuice,golang/go,ImJasonH/go,kim-racktop/go,goodlang/good,cyberroadie/go,akutz/go,ganboing/go-esx,cloudflare/go,binarycrusader/go,momchil-velikov/go,4ad/go,Cofyc/go,CAFxX/go,josharian/go,4ad/go,vanloswang/go,Cofyc/go,linux-on-ibm-z/go,vanloswang/go,odeke-em/go,Endika/go,kim-racktop/go,miolini/go,polyverse-security/go,momchil-velikov/go,miolini/go,suncycheng/go,Jordanzuo/go,vanloswang/go,polyverse-security/go,Jordanzuo/go,rachtsingh/gogojuice,ganboing/go-esx,binarycrusader/go,Jordanzuo/go,golang/go