runtime: stop using sigreturn on openbsd/386

In future releases of OpenBSD, the sigreturn syscall will no longer
exist. As such, stop using sigreturn on openbsd/386 and just return
from the signal trampoline (as we already do for openbsd/amd64 and
openbsd/arm).

Change-Id: Ic4de1795bbfbfb062a685832aea0d597988c6985
Reviewed-on: https://go-review.googlesource.com/23024Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/sys_openbsd_386.s

@@ -214,14 +214,6 @@ TEXT runtime·sigtramp(SB),NOSPLIT,$12
 	MOVL	context+8(FP), BX
 	MOVL	BX, 8(SP)
 	CALL	runtime·sigtrampgo(SB)
-
-	// call sigreturn
-	MOVL	context+8(FP), AX
-	MOVL	$0, 0(SP)		// syscall gap
-	MOVL	AX, 4(SP)		// arg 1 - sigcontext
-	MOVL	$103, AX		// sys_sigreturn
-	INT	$0x80
-	MOVL	$0xf1, 0xf1		// crash
 	RET
 
 // int32 tfork(void *param, uintptr psize, M *mp, G *gp, void (*fn)(void));