runtime: mark asmcgocall<>(SB) as having no arguments

It doesn't.
Fixes 386 build.

While we're here, mark runtime.asmcgocall as GO_ARGS,
so that it will work with stack copying. I don't think anything
that uses it can lead to a stack copy, but better safe than sorry.
Certainly the runtime.asmcgocall_errno variant needs
(and already has) GO_ARGS.

TBR=iant
CC=golang-codereviews
https://golang.org/cl/138400043

src/runtime/asm_386.s

@@ -647,6 +647,7 @@ TEXT gosave<>(SB),NOSPLIT,$0
 // aligned appropriately for the gcc ABI.
 // See cgocall.c for more details.
 TEXT runtime·asmcgocall(SB),NOSPLIT,$0-8
+	GO_ARGS
 	MOVL	fn+0(FP), AX
 	MOVL	arg+4(FP), BX
 	CALL	asmcgocall<>(SB)
@@ -660,7 +661,7 @@ TEXT runtime·asmcgocall_errno(SB),NOSPLIT,$0-12
 	MOVL	AX, ret+8(FP)
 	RET
 
-TEXT asmcgocall<>(SB),NOSPLIT,$0-12
+TEXT asmcgocall<>(SB),NOSPLIT,$0-0
 	// fn in AX, arg in BX
 	MOVL	SP, DX
 

src/runtime/asm_amd64.s

@@ -624,6 +624,7 @@ TEXT gosave<>(SB),NOSPLIT,$0
 // aligned appropriately for the gcc ABI.
 // See cgocall.c for more details.
 TEXT runtime·asmcgocall(SB),NOSPLIT,$0-16
+	GO_ARGS
 	MOVQ	fn+0(FP), AX
 	MOVQ	arg+8(FP), BX
 	CALL	asmcgocall<>(SB)

src/runtime/asm_arm.s

@@ -481,6 +481,7 @@ TEXT gosave<>(SB),NOSPLIT,$0
 // aligned appropriately for the gcc ABI.
 // See cgocall.c for more details.
 TEXT	runtime·asmcgocall(SB),NOSPLIT,$0-8
+	GO_ARGS
 	MOVW	fn+0(FP), R1
 	MOVW	arg+4(FP), R0
 	BL	asmcgocall<>(SB)