runtime: add stub sigreturn on arm

When building a shared library, all functions that are declared must actually
be defined.

Change-Id: I1488690cecfb66e62d9fdb3b8d257a4dc31d202a
Reviewed-on: https://go-review.googlesource.com/14187
Reviewed-by: Dave Cheney <dave@cheney.net>
diff --git a/src/runtime/asm_arm.s b/src/runtime/asm_arm.s
index 0659127..917cce8 100644
--- a/src/runtime/asm_arm.s
+++ b/src/runtime/asm_arm.s
@@ -1072,3 +1072,6 @@
 	MULU	R0, R1
 	SUB	R1, R3, R1
 	RET
+
+TEXT runtime·sigreturn(SB),NOSPLIT,$0-4
+        RET