internal/cpu: detect GFNI independently of AVX-512F

[alexrios]

GFNI is exposed in VEX-encoded form (VGF2P8AFFINEQB on XMM/YMM) on every
x86 CPU that reports the GFNI bit (CPUID leaf 7 ECX bit 8) and supports
YMM (HasAVX). Only the EVEX-encoded form requires AVX-512.

Since CL 655280 imported the Green Tea scan kernel, X86.HasGFNI was
populated inside the "if X86.HasAVX512F { ... }" block, leaving it false
on hosts that have VEX-256 GFNI but no AVX-512:

• Intel 12th-14th generation consumer CPUs (Alder/Raptor Lake) where
  AVX-512 is fuse-locked off to maintain ISA homogeneity with the
  E-cores.
• Intel Atom-derived SKUs (Jasper/Elkhart Lake, N100/N200/N300-series)
  which have shipped GFNI since Tremont but have never had AVX-512.

These platforms can execute VGF2P8AFFINEQB on YMM natively but the cpu
package incorrectly reported HasGFNI=false.

Move the standalone GFNI detection out of the AVX-512 block and gate it
on HasAVX (which requires OSXSAVE + YMM OS support), matching the
existing pattern for HasVAES. The EVEX form continues to be reported
separately via HasAVX512GFNI.

Add HasGFNI to the GODEBUG=cpu.* options table so users have a kill
switch on hosts the previous (over-restrictive) gating implicitly hid.
Without this, the only way to disable HasGFNI on a non-AVX-512 host
would be the sledgehammer GODEBUG=cpu.avx=off.

Add a regression test that reads CPUID leaf 7 ECX bit 8 directly and
asserts HasGFNI is set when the hardware bit is set and AVX is
available; verified to fail on the buggy code on an i9-14900K. Add a
paired TestDisableGFNI that exercises the new GODEBUG kill switch, and
TestX86ifGFNIhasAVX / TestX86ifAVX512GFNIhasGFNI invariant tests
following the established pattern in cpu_x86_test.go.

Fixes #79437

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Patch Set 1:

(1 comment)

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