runtime/cgo: restore the g pointer correctly in crosscall_s390x
runtime/cgo: restore the g pointer correctly in crosscall_s390x

R13 needs to be set to g because C code may have clobbered R13.

Fixes #16006.

Change-Id: I66311fe28440e85e589a1695fa1c42416583b4c6
Reviewed-on: https://go-review.googlesource.com/23910
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

derekmarcotte/go,theif519/go,linux-on-ibm-z/go,ImJasonH/go,derekmarcotte/go,ImJasonH/go,ImJasonH/go,polyverse-security/go,vsekhar/elastic-go,theif519/go,nawawi/go,vsekhar/elastic-go,josharian/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,Cofyc/go,theif519/go,suncycheng/go,golang/go,polyverse-security/go,akutz/go,CAFxX/go,vsekhar/elastic-go,goodlang/good,Cofyc/go,josharian/go,momchil-velikov/go,akutz/go,CAFxX/go,ImJasonH/go,josharian/go,goodlang/good,cloudflare/go,linux-on-ibm-z/go,momchil-velikov/go,golang/go,Cofyc/go,ImJasonH/go,suncycheng/go,linux-on-ibm-z/go,derekmarcotte/go,codestation/go,golang/go,akutz/go,codestation/go,cloudflare/go,theif519/go,polyverse-security/go,linux-on-ibm-z/go,theif519/go,CAFxX/go,theif519/go,derekmarcotte/go,polyverse-security/go,CAFxX/go,Cofyc/go,vsekhar/elastic-go,4ad/go,4ad/go,momchil-velikov/go,christopher-henderson/Go,cloudflare/go,vsekhar/elastic-go,goodlang/good,nawawi/go,cloudflare/go,codestation/go,derekmarcotte/go,derekmarcotte/go,suncycheng/go,codestation/go,nawawi/go,momchil-velikov/go,linux-on-ibm-z/go,josharian/go,golang/go,golang/go,Cofyc/go,suncycheng/go,theif519/go,goodlang/good,linux-on-ibm-z/go,polyverse-security/go,momchil-velikov/go,akutz/go,vsekhar/elastic-go,4ad/go,momchil-velikov/go,nawawi/go,4ad/go,cloudflare/go,josharian/go,CAFxX/go,polyverse-security/go,codestation/go,Cofyc/go,momchil-velikov/go,Cofyc/go,linux-on-ibm-z/go,suncycheng/go,vsekhar/elastic-go,4ad/go,CAFxX/go,akutz/go,4ad/go,codestation/go,derekmarcotte/go,derekmarcotte/go,goodlang/good,polyverse-security/go,nawawi/go,josharian/go,vsekhar/elastic-go,josharian/go,cloudflare/go,suncycheng/go,akutz/go,linux-on-ibm-z/go,Cofyc/go,polyverse-security/go,josharian/go,ImJasonH/go,codestation/go,ImJasonH/go,golang/go,theif519/go,suncycheng/go,akutz/go,akutz/go,nawawi/go,goodlang/good,goodlang/good,codestation/go,goodlang/good,4ad/go,suncycheng/go,CAFxX/go,momchil-velikov/go,cloudflare/go,nawawi/go,golang/go,4ad/go,ImJasonH/go