cmd/internal/ld: generate correct .debug_frames on RISC architectures

With this patch, gdb seems to be able to corretly backtrace Go
process on at least linux/{arm,arm64,ppc64}.

Change-Id: Ic40a2a70e71a19c4a92e4655710f38a807b67e9a
Reviewed-on: https://go-review.googlesource.com/9822
Run-TryBot: Minux Ma <minux@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/ld/lib.go

@@ -86,6 +86,7 @@ type Arch struct {
 	Maxalign         int
 	Minlc            int
 	Dwarfregsp       int
+	Dwarfreglr       int
 	Linuxdynld       string
 	Freebsddynld     string
 	Netbsddynld      string