cmd/compile: intrinsify runtime/internal/atomic.Xaddint64
cmd/compile: intrinsify runtime/internal/atomic.Xaddint64

This aliases to Xadd64.

Change-Id: I95d49e1d03eecc242e9e6fd4b2742b1c1a1d5ade
Reviewed-on: https://go-review.googlesource.com/29274
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>

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