cmd/link: add -h flag, for debugging, same as cmd/compile

Change-Id: I3c9b05879fe0b6e94b63e9b65e4411ba2a917134
Reviewed-on: https://go-review.googlesource.com/10705Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -1949,8 +1949,10 @@ func Diag(format string, args ...interface{}) {
 		sep = ": "
 	}
 	fmt.Printf("%s%s%s\n", tn, sep, fmt.Sprintf(format, args...))
-
 	nerrors++
+	if Debug['h'] != 0 {
+		panic("error")
+	}
 	if nerrors > 20 {
 		Exitf("too many errors")
 	}

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

@@ -111,6 +111,7 @@ func Ldmain() {
 	obj.Flagstr("extldflags", "pass `flags` to external linker", &extldflags)
 	obj.Flagcount("f", "ignore version mismatch", &Debug['f'])
 	obj.Flagcount("g", "disable go package data checks", &Debug['g'])
+	obj.Flagcount("h", "halt on error", &Debug['h'])
 	obj.Flagstr("installsuffix", "set package directory `suffix`", &flag_installsuffix)
 	obj.Flagstr("k", "set field tracking `symbol`", &tracksym)
 	obj.Flagfn1("linkmode", "set link `mode` (internal, external, auto)", setlinkmode)