cmd/go: recognise plan9_arm object files

Add magic word for Plan 9 ARM object header to objectMagic table.

Change-Id: I21eb8845a2ee2e8cdddc0849eedf43481aee9cde
Reviewed-on: https://go-review.googlesource.com/18963Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/build.go

@@ -1835,6 +1835,7 @@ var objectMagic = [][]byte{
 	{0x4d, 0x5a, 0x90, 0x00, 0x03, 0x00},      // PE (Windows) as generated by 6l/8l and gcc
 	{0x00, 0x00, 0x01, 0xEB},                  // Plan 9 i386
 	{0x00, 0x00, 0x8a, 0x97},                  // Plan 9 amd64
+	{0x00, 0x00, 0x06, 0x47},                  // Plan 9 arm
 }
 
 func isObject(s string) bool {