[dev.cc] cmd/asm/internal/asm: fix test on nacl

Need to add testdata to NaCl's testing file system.

Change-Id: Ie9703b5475c7f87e737a06de85d1f88e2062f090
Reviewed-on: https://go-review.googlesource.com/5541Reviewed-by: Russ Cox <rsc@golang.org>

misc/nacl/testzip.proto

@@ -10,6 +10,11 @@ usr	src=../misc/nacl/testdata
 go	src=..
 	src
 		cmd
+			asm
+				internal
+					asm
+						testdata
+							+
 			internal
 				objfile
 					objfile.go