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/14187Reviewed-by: Dave Cheney <dave@cheney.net>

src/runtime/asm_arm.s

@@ -1072,3 +1072,6 @@ TEXT runtime·usplitR0(SB),NOSPLIT,$0
 	MULU	R0, R1
 	SUB	R1, R3, R1
 	RET
+
+TEXT runtime·sigreturn(SB),NOSPLIT,$0-4
+        RET