cmd/objdump: set goarch properly on non-386 Plan 9 systems

LGTM=0intro, r
R=0intro, r
CC=ality, golang-codereviews, jas, mischief
https://golang.org/cl/108420043

src/cmd/objdump/plan9obj.go

@@ -34,7 +34,14 @@ func plan9Symbols(f *os.File) (syms []Sym, goarch string) {
 		return
 	}
 
-	goarch = "386"
+	switch p.Magic {
+	case plan9obj.MagicAMD64:
+		goarch = "amd64"
+	case plan9obj.Magic386:
+		goarch = "386"
+	case plan9obj.MagicARM:
+		goarch = "arm"
+	}
 
 	// Build sorted list of addresses of all symbols.
 	// We infer the size of a symbol by looking at where the next symbol begins.