cmd/vet: fix line number in asm errors
cmd/vet: fix line number in asm errors

Before:
: asm_amd64.s:16: [amd64] invalid offset new+16(FP); expected new+8(FP)
: asm_amd64.s:26: [amd64] invalid offset new+16(FP); expected new+8(FP)

After:
asm_amd64.s:16: [amd64] invalid offset new+16(FP); expected new+8(FP)
asm_amd64.s:26: [amd64] invalid offset new+16(FP); expected new+8(FP)

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/91510043

t-yuki/go.tools.old,t-yuki/go.tools.old,t-yuki/go.tools.old