runtime: get rid of SA_RESTORER on ARM.

The manpages says SA_RESTORER is obsolete, and indeed, not every architecture
support it. However, sadly it's required on x86_64, see http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/signal.c?id=26bcd8b72563b4c54892c4c2a409f6656fb8ae8b#n430, so only use it on x86.

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://golang.org/cl/115450043

src/pkg/runtime/defs_linux_arm.h

@@ -16,7 +16,7 @@ enum {
 	MADV_DONTNEED = 0x4,
 	SA_RESTART = 0x10000000,
 	SA_ONSTACK = 0x8000000,
-	SA_RESTORER = 0x4000000,
+	SA_RESTORER = 0, // unused on ARM
 	SA_SIGINFO = 0x4,
 	SIGHUP = 0x1,
 	SIGINT = 0x2,

src/pkg/runtime/os_linux.c

@@ -288,7 +288,7 @@ runtime·memlimit(void)
  * and calls sighandler().
  */
 extern void runtime·sigtramp(void);
-extern void runtime·sigreturn(void);	// calls runtime·sigreturn
+extern void runtime·sigreturn(void);	// calls rt_sigreturn, only used with SA_RESTORER
 
 void
 runtime·setsig(int32 i, GoSighandler *fn, bool restart)
@@ -300,9 +300,15 @@ runtime·setsig(int32 i, GoSighandler *fn, bool restart)
 	if(restart)
 		sa.sa_flags |= SA_RESTART;
 	sa.sa_mask = ~0ULL;
-	// TODO(adonovan): Linux manpage says "sa_restorer element is
-	// obsolete and should not be used".  Avoid it here, and test.
+	// Although Linux manpage says "sa_restorer element is obsolete and
+	// should not be used". x86_64 kernel requires it. Only use it on
+	// x86.
+#ifdef GOARCH_386
+	sa.sa_restorer = (void*)runtime·sigreturn;
+#endif
+#ifdef GOARCH_amd64
 	sa.sa_restorer = (void*)runtime·sigreturn;
+#endif
 	if(fn == runtime·sighandler)
 		fn = (void*)runtime·sigtramp;
 	sa.sa_handler = fn;

src/pkg/runtime/sys_linux_arm.s

@@ -340,11 +340,6 @@ TEXT runtime·rt_sigaction(SB),NOSPLIT,$0
 	SWI	$0
 	RET
 
-TEXT runtime·sigreturn(SB),NOSPLIT,$0
-	MOVW	$SYS_rt_sigreturn, R7
-	SWI	$0
-	RET
-
 TEXT runtime·usleep(SB),NOSPLIT,$12
 	MOVW	usec+0(FP), R0
 	MOVW	R0, R1