runtime: two trivial but important bug fixes

R=r
https://golang.org/cl/156059

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

@@ -287,7 +287,7 @@ TEXT setldt(SB),7,$32
 	ORL	$0x40, CX		// 32-bit operand size
 	MOVB	CX, 6(AX)
 
-	MOVL	$0xF2, 5(AX)	// r/w data descriptor, dpl=3, present
+	MOVB	$0xF2, 5(AX)	// r/w data descriptor, dpl=3, present
 
 	// call i386_set_ldt(entry, desc, 1)
 	MOVL	$0xffffffff, 0(SP)	// auto-allocate entry and return in AX

src/pkg/runtime/proc.c

@@ -413,7 +413,7 @@ matchmg(void)
 {
 	G *g;
 
-	if(m->mallocing)
+	if(m->mallocing || m->gcing)
 		return;
 	while(sched.mcpu < sched.mcpumax && (g = gget()) != nil){
 		M *m;