cmd/compile, cmd/link: eliminate uses of ArchFamily in error messages

Two of these error messages are already dead code: cmd/compile.main
and cmd/link.main already switch on $GOARCH, ensuring it must be a
prefix of the sys.Arch.Family.

The error message about uncompiled Go source files can be just be
simplified: anyone who's manually constructing Go object file archives
probably knows what tool to use to compile Go source files.

Change-Id: Ia4a67c0a1d1158379c127c91e909226d3367f3c2
Reviewed-on: https://go-review.googlesource.com/21626
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/main.go

@@ -93,14 +93,7 @@ func doversion() {
 func Main() {
 	defer hidePanic()
 
-	// Allow GOARCH=thearch.thestring or GOARCH=thearch.thestringsuffix,
-	// but not other values.
-	p := obj.Getgoarch()
-
-	if !strings.HasPrefix(p, Thearch.LinkArch.Name) {
-		log.Fatalf("cannot use %cg with GOARCH=%s", Thearch.LinkArch.Family, p)
-	}
-	goarch = p
+	goarch = obj.Getgoarch()
 
 	Ctxt = obj.Linknew(Thearch.LinkArch)
 	Ctxt.DiagFunc = Yyerror

src/cmd/link/internal/ld/lib.go

@@ -1301,7 +1301,8 @@ func ldobj(f *obj.Biobuf, pkg string, length int64, pn string, file string, when
 
 	if !strings.HasPrefix(line, "go object ") {
 		if strings.HasSuffix(pn, ".go") {
-			Exitf("%cl: input %s is not .%c file (use %cg to compile .go files)", SysArch.Family, pn, SysArch.Family, SysArch.Family)
+			Exitf("%s: uncompiled .go source file", pn)
+			return nil
 		}
 
 		if line == SysArch.Name {
@@ -1559,10 +1560,6 @@ func mywhatsys() {
 	goroot = obj.Getgoroot()
 	goos = obj.Getgoos()
 	goarch = obj.Getgoarch()
-
-	if !strings.HasPrefix(goarch, SysArch.Name) {
-		log.Fatalf("cannot use %cc with GOARCH=%s", SysArch.Family, goarch)
-	}
 }
 
 // Copied from ../gc/subr.c:/^pathtoprefix; must stay in sync.