cmd/asm: reject misplaced go:build comments

We are converting from using error-prone ad-hoc syntax // +build lines
to less error-prone, standard boolean syntax //go:build lines.
The timeline is:

Go 1.16: prepare for transition
 - Builds still use // +build for file selection.
 - Source files may not contain //go:build without // +build.
 - Builds fail when a source file contains //go:build lines without // +build lines. <<<

Go 1.17: start transition
 - Builds prefer //go:build for file selection, falling back to // +build
   for files containing only // +build.
 - Source files may contain //go:build without // +build (but they won't build with Go 1.16).
 - Gofmt moves //go:build and // +build lines to proper file locations.
 - Gofmt introduces //go:build lines into files with only // +build lines.
 - Go vet rejects files with mismatched //go:build and // +build lines.

Go 1.18: complete transition
 - Go fix removes // +build lines, leaving behind equivalent // +build lines.

This CL provides part of the <<< marked line above in the Go 1.16 step:
rejecting files containing //go:build but not // +build.

Reject any //go:build comments found after actual assembler code
(include #include etc directives), because the go command itself
doesn't read that far.

For #41184.

Change-Id: Ib460bfd380cce4239993980dd208afd07deff3f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/240602
Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/asm/internal/lex/input.go

@@ -109,6 +109,9 @@ func (in *Input) Next() ScanToken {
 				in.Error("'#' must be first item on line")
 			}
 			in.beginningOfLine = in.hash()
+			in.text = "#"
+			return '#'
+
 		case scanner.Ident:
 			// Is it a macro name?
 			name := in.Stack.Text()