runtime: fix usleep on linux/386 and re-enable parallel gc

R=golang-dev, jsing, alex.brainman, cw, rsc
CC=golang-dev
https://golang.org/cl/5166047

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

@@ -52,23 +52,22 @@ TEXT runtime·read(SB),7,$0
 	CALL	*runtime·_vdso(SB)
 	RET
 
-TEXT runtime·usleep(SB),7,$28
+TEXT runtime·usleep(SB),7,$8
 	MOVL	$0, DX
 	MOVL	usec+0(FP), AX
 	MOVL	$1000000, CX
 	DIVL	CX
-	MOVL	AX, 20(SP)
-	MOVL	DX, 24(SP)
+	MOVL	AX, 0(SP)
+	MOVL	DX, 4(SP)
 
 	// select(0, 0, 0, 0, &tv)
-	MOVL	$0, 0(SP)
-	MOVL	$0, 4(SP)
-	MOVL	$0, 8(SP)
-	MOVL	$0, 12(SP)
-	LEAL	20(SP), AX
-	MOVL	AX, 16(SP)
-	MOVL	$82, AX
-	SYSCALL
+	MOVL	$142, AX
+	MOVL	$0, BX
+	MOVL	$0, CX
+	MOVL	$0, DX
+	MOVL	$0, SI
+	LEAL	0(SP), DI
+	CALL	*runtime·_vdso(SB)
 	RET
 
 TEXT runtime·raisesigpipe(SB),7,$12

src/pkg/runtime/proc.c

@@ -607,7 +607,6 @@ runtime·helpgc(void)
 	M *m;
 	int32 n, max;
 
-return 0;  // broken on linux/386
 	// Figure out how many CPUs to use.
 	// Limited by gomaxprocs, number of actual CPUs, and MaxGcproc.
 	max = runtime·gomaxprocs;
@@ -616,7 +615,6 @@ return 0;  // broken on linux/386
 	if(max > MaxGcproc)
 		max = MaxGcproc;
 
-
 	// We're going to use one CPU no matter what.
 	// Figure out the max number of additional CPUs.
 	max--;