darwin: bsdthread_create can fail; print good error

Fixes #549.

R=adg
CC=golang-dev
https://golang.org/cl/1019042

src/pkg/runtime/darwin/386/sys.s

@@ -130,8 +130,10 @@ TEXT bsdthread_create(SB),7,$32
 	MOVL	BX, 16(SP)	// pthread
 	MOVL	$0x1000000, 20(SP)	// flags = PTHREAD_START_CUSTOM
 	INT	$0x80
-	JAE	2(PC)
-	CALL	notok(SB)
+	JAE	3(PC)
+	MOVL	$-1, AX
+	RET
+	MOVL	$0, AX
 	RET
 
 // The thread that bsdthread_create creates starts executing here,

src/pkg/runtime/darwin/amd64/sys.s

@@ -116,8 +116,10 @@ TEXT bsdthread_create(SB),7,$0
 	MOVQ	$0, R9	// paranoia
 	MOVQ	$(0x2000000+360), AX	// bsdthread_create
 	SYSCALL
-	JCC 2(PC)
-	CALL	notok(SB)
+	JCC 3(PC)
+	MOVL	$-1, AX
+	RET
+	MOVL	$0, AX
 	RET
 
 // The thread that bsdthread_create creates starts executing here,

src/pkg/runtime/darwin/os.h

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-void	bsdthread_create(void*, M*, G*, void(*)(void));
+int32	bsdthread_create(void*, M*, G*, void(*)(void));
 void	bsdthread_register(void);
 int32	mach_msg_trap(MachHeader*, int32, uint32, uint32, uint32, uint32, uint32);
 uint32	mach_reply_port(void);

src/pkg/runtime/darwin/thread.c

@@ -157,7 +157,8 @@ newosproc(M *m, G *g, void *stk, void (*fn)(void))
 		printf("newosproc stk=%p m=%p g=%p fn=%p id=%d/%d ostk=%p\n",
 			stk, m, g, fn, m->id, m->tls[0], &m);
 	}
-	bsdthread_create(stk, m, g, fn);
+	if(bsdthread_create(stk, m, g, fn) < 0)
+		throw("cannot create new OS thread");
 }
 
 // Called to initialize a new m (including the bootstrap m).