internal/bytealg: remove dependency on runtime·support_avx2

Use internal/cpu instead.

Change-Id: I8670440389cbd88951fee61e352c4a10ac7eee6e
Reviewed-on: https://go-review.googlesource.com/102737
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/internal/bytealg/count_amd64.s

@@ -151,7 +151,7 @@ endofpage:
 	RET
 
 avx2:
-	CMPB   runtime·support_avx2(SB), $1
+	CMPB   internal∕cpu·X86+const_x86_HasAVX2(SB), $1
 	JNE sse
 	MOVD AX, X0
 	LEAQ -32(SI)(BX*1), R11