runtime: tweaks to allow -buildmode=shared to work

Building Go shared libraries requires that all functions that have declarations
without bodies have implementations and vice versa, so remove the
implementation of call16 and add a stub implementation of sigreturn.

Change-Id: I4d5a30c8637a5da7991054e151a536611d5bea46
Reviewed-on: https://go-review.googlesource.com/15966Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/asm_ppc64x.s

@@ -426,7 +426,6 @@ end:						\
 	BL	runtime·callwritebarrier(SB);	\
 	RET
 
-CALLFN(·call16, 16)
 CALLFN(·call32, 32)
 CALLFN(·call64, 64)
 CALLFN(·call128, 128)
@@ -1227,3 +1226,6 @@ TEXT runtime·prefetcht2(SB),NOSPLIT,$0-8
 
 TEXT runtime·prefetchnta(SB),NOSPLIT,$0-8
 	RET
+
+TEXT runtime·sigreturn(SB),NOSPLIT,$0-8
+        RET