runtime: jump to badmcall instead of calling it.

This replaces the mcall frame with the badmcall frame instead of
leaving the mcall frame on the stack and adding the badmcall frame.
Because mcall is no longer on the stack, traceback will now report what
called mcall, which is what we would like to see in this situation.

R=golang-dev, cshapiro
CC=golang-dev
https://golang.org/cl/13012044

src/pkg/runtime/asm_386.s

@@ -181,14 +181,16 @@ TEXT runtime·mcall(SB), NOSPLIT, $0-4
 	MOVL	m(CX), BX
 	MOVL	m_g0(BX), SI
 	CMPL	SI, AX	// if g == m->g0 call badmcall
-	JNE	2(PC)
-	CALL	runtime·badmcall(SB)
+	JNE	3(PC)
+	MOVL	$runtime·badmcall(SB), AX
+	JMP	AX
 	MOVL	SI, g(CX)	// g = m->g0
 	MOVL	(g_sched+gobuf_sp)(SI), SP	// sp = m->g0->sched.sp
 	PUSHL	AX
 	CALL	DI
 	POPL	AX
-	CALL	runtime·badmcall2(SB)
+	MOVL	$runtime·badmcall2(SB), AX
+	JMP	AX
 	RET
 
 /*

src/pkg/runtime/asm_amd64.s

@@ -169,16 +169,16 @@ TEXT runtime·mcall(SB), NOSPLIT, $0-8
 	MOVQ	m_g0(BX), SI
 	CMPQ	SI, AX	// if g == m->g0 call badmcall
 	JNE	3(PC)
-	ARGSIZE(0)
-	CALL	runtime·badmcall(SB)
+	MOVQ	$runtime·badmcall(SB), AX
+	JMP	AX
 	MOVQ	SI, g(CX)	// g = m->g0
 	MOVQ	(g_sched+gobuf_sp)(SI), SP	// sp = m->g0->sched.sp
 	PUSHQ	AX
 	ARGSIZE(8)
 	CALL	DI
 	POPQ	AX
-	ARGSIZE(0)
-	CALL	runtime·badmcall2(SB)
+	MOVQ	$runtime·badmcall2(SB), AX
+	JMP	AX
 	RET
 
 /*

src/pkg/runtime/asm_arm.s

@@ -157,12 +157,13 @@ TEXT runtime·mcall(SB), NOSPLIT, $-4-4
 	MOVW	g, R1
 	MOVW	m_g0(m), g
 	CMP	g, R1
-	BL.EQ	runtime·badmcall(SB)
+	B.NE	2(PC)
+	B	runtime·badmcall(SB)
 	MOVW	(g_sched+gobuf_sp)(g), SP
 	SUB	$8, SP
 	MOVW	R1, 4(SP)
 	BL	(R0)
-	BL	runtime·badmcall2(SB)
+	B	runtime·badmcall2(SB)
 	RET
 
 /*

src/pkg/runtime/proc.c

@@ -1997,14 +1997,16 @@ runtime·mcount(void)
 }
 
 void
-runtime·badmcall(void)  // called from assembly
+runtime·badmcall(void (*fn)(G*))  // called from assembly
 {
+	USED(fn); // TODO: print fn?
 	runtime·throw("runtime: mcall called on m->g0 stack");
 }
 
 void
-runtime·badmcall2(void)  // called from assembly
+runtime·badmcall2(void (*fn)(G*))  // called from assembly
 {
+	USED(fn);
 	runtime·throw("runtime: mcall function returned");
 }