cmd/link: no dwarf on darwin/arm

Partial revert of cl/10284 to get -buildmode=c-archive working for
darwin/arm.

Manually tested with iostest.bash while builder is offline.

Change-Id: I98e4e209765666e320e680e11151fce59e2afde9
Reviewed-on: https://go-review.googlesource.com/11306Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>

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

@@ -444,9 +444,13 @@ func Asmbmacho() {
 		ms = newMachoSeg("", 40)
 
 		ms.fileoffset = Segtext.Fileoff
+		if Thearch.Thechar == '5' {
+			ms.filesize = Segdata.Fileoff + Segdata.Filelen - Segtext.Fileoff
+		} else {
 			ms.filesize = Segdwarf.Fileoff + Segdwarf.Filelen - Segtext.Fileoff
 			ms.vsize = ms.filesize
 		}
+	}
 
 	/* segment for zero page */
 	if Linkmode != LinkExternal {