runtime/internal/atomic: Remove unnecessary checks for GOARCH_ppc64
runtime/internal/atomic: Remove unnecessary checks for GOARCH_ppc64

Starting in go1.9, the minimum processor requirement for ppc64 is POWER8. This
means the checks for GOARCH_ppc64 in asm_ppc64x.s can be removed, since we can
assume LBAR and STBCCC instructions (both from ISA 2.06) will always be
available.

Updates #19074

Change-Id: Ib4418169cd9fc6f871a5ab126b28ee58a2f349e2
Reviewed-on: https://go-review.googlesource.com/38406
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Lynn Boger <b27445fe6fb259c37b8e0a4ce56e22e2413754bf@linux.vnet.ibm.com>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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