cmd/vet: +build comment error is confusingly worded

Please answer these questions before submitting your issue. Thanks!

What did you do?

Hello up there. I was updating my tracing/xruntime package for Go1.12 and hit test error:

.../src/lab.nexedi.com/kirr/go123/tracing/internal/xruntime$ go test
# lab.nexedi.com/kirr/go123/tracing/internal/xruntime
./runtime_g_amd64.s:3:1: +build comment must appear before package clause and be followed by a blank line
FAIL    lab.nexedi.com/kirr/go123/tracing/internal/xruntime [build failed]

The error here complains about +build directive in assembly file:

---- 8< ---- (runtime_g_amd64.s)

#include "textflag.h"

// +build amd64 amd64p

// func getg() *g
TEXT ·getg(SB),NOSPLIT,$0-8
	MOVQ (TLS), R14
	MOVQ R14, ret+0(FP)
	RET

(https://lab.nexedi.com/kirr/go123/blob/7ee2de42/tracing/internal/xruntime/runtime_g_amd64.s)

It was working with Go1.11 and previous releases.

What did you expect to see?

Build and test succeed; test pass, as with e.g. Go1.11:

.../src/lab.nexedi.com/kirr/go123/tracing/internal/xruntime$ go1.11 test -v
=== RUN   TestStartStopTheWorld
--- PASS: TestStartStopTheWorld (1.00s)
PASS
ok      lab.nexedi.com/kirr/go123/tracing/internal/xruntime     1.003s

What did you see instead?

.../src/lab.nexedi.com/kirr/go123/tracing/internal/xruntime$ go1.12 test -v
# lab.nexedi.com/kirr/go123/tracing/internal/xruntime
./runtime_g_amd64.s:3:1: +build comment must appear before package clause and be followed by a blank line
FAIL    lab.nexedi.com/kirr/go123/tracing/internal/xruntime [build failed]

System details

go version go1.12.3 linux/amd64
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/kirr/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/kirr/go"
GOPROXY=""
GORACE=""
GOROOT="/home/kirr/src/tools/go/go"
GOTMPDIR=""
GOTOOLDIR="/home/kirr/src/tools/go/go/pkg/tool/linux_amd64"
GCCGO="/usr/bin/gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
GOROOT/bin/go version: go version go1.12.3 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.12.3
uname -sr: Linux 4.19.0-4-amd64
Distributor ID:	Debian
Description:	Debian GNU/Linux buster/sid
Release:	testing
Codename:	buster
/lib/x86_64-linux-gnu/libc.so.6: GNU C Library (Debian GLIBC 2.28-8) stable release version 2.28.
gdb --version: GNU gdb (Debian 8.2.1-2) 8.2.1