[dev.link] cmd/asm: new -p option, changes to DWARF generation

Adds a new "-p" option to the assembler, for specifying the import
path of the package being compiled. DWARF generation is now conditional
on having a valid package path -- if we don't know the package path,
then don't emit DWARF.

This is intended to lay the groundwork for removing the various
"patchDWARFname" hacks in the linker.

Change-Id: I5f8315c0881791eb8fe1f2ba32f5bb0ae76f6b98
Reviewed-on: https://go-review.googlesource.com/c/go/+/222718
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

src/cmd/asm/main.go

@@ -73,7 +73,7 @@ func main() {
 			pList.Firstpc, ok = parser.Parse()
 			// reports errors to parser.Errorf
 			if ok {
-				obj.Flushplist(ctxt, pList, nil, "")
+				obj.Flushplist(ctxt, pList, nil, *flags.Importpath)
 			}
 		}
 		if !ok {