cpu: remove the declaration of function darwinSupportsAVX512

The implement of darwinSupportsAVX512 has been removed in CL 361255.
But the command "go install -buildmode=shared std" failed because the
declaration of it is remained in cpu/cpu_gc_x86.go. It should be removed.

Update golang/go#49233
For golang/go#49942

Change-Id: I8fa7c61c20457e49414930029b9f026c335aa421
Reviewed-on: https://go-review.googlesource.com/c/sys/+/368994
Reviewed-by: Tobias Klauser <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Trust: Ian Lance Taylor <[email protected]>

Author: zhouguangyuan0718

cpu/cpu_gc_x86.go

@@ -15,7 +15,3 @@ func cpuid(eaxArg, ecxArg uint32) (eax, ebx, ecx, edx uint32)
 // xgetbv with ecx = 0 is implemented in cpu_x86.s for gc compiler
 // and in cpu_gccgo.c for gccgo.
 func xgetbv() (eax, edx uint32)
-
-// darwinSupportsAVX512 is implemented in cpu_x86.s for gc compiler
-// and in cpu_gccgo_x86.go for gccgo.
-func darwinSupportsAVX512() bool