cmd/compile: don't Fatal when printing -m debug information

Some FmtSharp export formatting flag support was removed with
commit b4e9f70. Don't panic if FmtSharp is set, just ignore it.

Fixes #16820.

Change-Id: Ie0c3d3774bd55002f6d2781b1212d070f083e6b2
Reviewed-on: https://go-review.googlesource.com/27556
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -113,8 +113,7 @@ func setfmode(flags *FmtFlag) (fm int, fb bool) {
 	if *flags&FmtSign != 0 {
 		fmtmode = FDbg
 	} else if *flags&FmtSharp != 0 {
-		// for textual export format - no longer supported
-		Fatalf("textual export format request")
+		// ignore (textual export format no longer supported)
 	} else if *flags&FmtLeft != 0 {
 		fmtmode = FTypeId
 	}