cmd/vet/all: add countBytes exemption to amd64 whitelist

Fixes vetall build broken by CL 38693.

Change-Id: I5c98f2164e0c7bb43f022c363b3b7c0650c4bcbe
Reviewed-on: https://go-review.googlesource.com/39916
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/all/whitelist/amd64.txt

@@ -36,4 +36,4 @@ runtime/duff_amd64.s: [amd64] duffzero: function duffzero missing Go declaration
 runtime/duff_amd64.s: [amd64] duffcopy: function duffcopy missing Go declaration
 runtime/asm_amd64.s: [amd64] stackcheck: function stackcheck missing Go declaration
 runtime/asm_amd64.s: [amd64] indexShortStr: function indexShortStr missing Go declaration
-
+runtime/asm_amd64.s: [amd64] countByte: function countByte missing Go declaration