runtime: more build fixing

Move the mstartfn into its own field.
Simpler, more likely to be correct.

R=golang-dev, devon.odell
CC=golang-dev
https://golang.org/cl/7414046

src/pkg/runtime/runtime.h

@@ -265,7 +265,8 @@ struct	M
 	uintptr	cret;		// return value from C
 	uint64	procid;		// for debuggers, but offset not hard-coded
 	G*	gsignal;	// signal-handling G
-	uint64	tls[4];		// thread-local storage (for x86 extern register)
+	uintptr	tls[4];		// thread-local storage (for x86 extern register)
+	void	(*mstartfn)(void);
 	G*	curg;		// current running goroutine
 	P*	p;		// attached P for executing Go code (nil if not executing Go code)
 	P*	nextp;

src/pkg/runtime/sys_freebsd_386.s

@@ -39,9 +39,10 @@ TEXT runtime·thr_start(SB),7,$0
 	MOVL	AX, m(CX)
 	CALL	runtime·stackcheck(SB)		// smashes AX
 
-	// newosproc left the function we should call in mp->tls[2] for us.
+	// newosproc left the function we should call in mp->mstartfn.
 	get_tls(CX)
-	MOVQ	8(CX), AX
+	MOVL	m(CX), AX
+	MOVL	m_mstartfn(AX), AX
 	CALL	AX
 
 	MOVL	0, AX			// crash (not reached)

src/pkg/runtime/sys_freebsd_amd64.s

@@ -34,16 +34,16 @@ TEXT runtime·thr_start(SB),7,$0
 
 	// set up m, g
 	get_tls(CX)
-	MOVQ	8(CX), AX
 	MOVQ	R13, m(CX)
 	MOVQ	m_g0(R13), DI
 	MOVQ	DI, g(CX)
 
 	CALL runtime·stackcheck(SB)
 	
-	// newosproc left the function we should call in mp->tls[2] for us.
+	// newosproc left the function we should call in mp->mstartfn.
 	get_tls(CX)
-	MOVQ	16(CX), AX
+	MOVQ	m(CX), AX
+	MOVQ	m_mstartfn(AX), AX
 	CALL	AX
 
 	MOVQ 0, AX			// crash (not reached)

src/pkg/runtime/sys_windows_386.s

@@ -260,9 +260,10 @@ TEXT runtime·tstart(SB),7,$0
 
 	CALL	runtime·stackcheck(SB)	// clobbers AX,CX
 
-	// start function is in tls[2]
+	// newosproc left the function we should call in mp->mstartfn.
 	get_tls(CX)
-	MOVL	8(CX), AX
+	MOVL	m(CX), AX
+	MOVL	m_mstartfn(AX), AX
 	CALL	AX
 
 	RET

src/pkg/runtime/sys_windows_amd64.s

@@ -330,9 +330,9 @@ TEXT runtime·tstart_stdcall(SB),7,$0
 
 	CALL	runtime·stackcheck(SB)	// clobbers AX,CX
 
-	// start function is in tls[2]
 	get_tls(CX)
-	MOVQ	16(CX), AX
+	MOVQ	m(CX), AX
+	MOVQ	m_mstartfn(AX), AX
 	CALL	AX
 
 	XORL	AX, AX			// return 0 == success

src/pkg/runtime/thread_freebsd.c

@@ -104,7 +104,7 @@ runtime·newosproc(M *mp, G *gp, void *stk, void (*fn)(void))
 	param.tls_size = sizeof mp->tls;
 
 	mp->tls[0] = mp->id;	// so 386 asm can find it
-	mp->tls[2] = (uintptr)fn;
+	mp->mstartfn = fn;
 
 	runtime·thr_new(&param, sizeof param);
 	runtime·sigprocmask(&oset, nil);

src/pkg/runtime/thread_windows.c

@@ -197,7 +197,7 @@ runtime·newosproc(M *mp, G *gp, void *stk, void (*fn)(void))
 	if(gp != mp->g0)
 		runtime·throw("invalid newosproc gp");
 
-	mp->tls[2] = (uintptr)fn;
+	mp->mstartfn = fn;
 
 	thandle = runtime·stdcall(runtime·CreateThread, 6,
 		nil, (uintptr)0x20000, runtime·tstart_stdcall, mp,