runtime: set runtime ncpu on openbsd

Set the runtime ncpu based on the hw.ncpu sysctl.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5191043

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

@@ -269,4 +269,22 @@ TEXT runtime·osyield(SB),7,$-4
 	INT	$0x80
 	RET
 
+TEXT runtime·sysctl(SB),7,$28
+	LEAL	arg0+0(FP), SI
+	LEAL	4(SP), DI
+	CLD
+	MOVSL				// arg 1 - name
+	MOVSL				// arg 2 - namelen
+	MOVSL				// arg 3 - oldp
+	MOVSL				// arg 4 - oldlenp
+	MOVSL				// arg 5 - newp
+	MOVSL				// arg 6 - newlen
+	MOVL	$202, AX		// sys___sysctl
+	INT	$0x80
+	JCC	3(PC)
+	NEGL	AX
+	RET
+	MOVL	$0, AX
+	RET
+
 GLOBL runtime·tlsoffset(SB),$4

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

@@ -235,3 +235,19 @@ TEXT runtime·settls(SB),7,$8
 	JCC	2(PC)
 	CALL	runtime·notok(SB)
 	RET
+
+TEXT runtime·sysctl(SB),7,$0
+	MOVQ	8(SP), DI		// arg 1 - name
+	MOVL	16(SP), SI		// arg 2 - namelen
+	MOVQ	24(SP), DX		// arg 3 - oldp
+	MOVQ	32(SP), R10		// arg 4 - oldlenp
+	MOVQ	40(SP), R8		// arg 5 - newp
+	MOVQ	48(SP), R9		// arg 6 - newlen
+	MOVQ	$202, AX		// sys___sysctl
+	SYSCALL
+	JCC 3(PC)
+	NEGL	AX
+	RET
+	MOVL	$0, AX
+	RET
+

src/pkg/runtime/openbsd/os.h

@@ -8,5 +8,6 @@ void	runtime·sigaltstack(Sigaltstack*, Sigaltstack*);
 void	runtime·sigaction(int32, struct sigaction*, struct sigaction*);
 void	runtime·setitimerval(int32, Itimerval*, Itimerval*);
 void	runtime·setitimer(int32, Itimerval*, Itimerval*);
+int32	runtime·sysctl(uint32*, uint32, byte*, uintptr*, byte*, uintptr);
 
 void	runtime·raisesigpipe(void);

src/pkg/runtime/openbsd/thread.c

@@ -15,6 +15,30 @@ enum
 	ENOTSUP = 91,
 };
 
+// From OpenBSD's <sys/sysctl.h>
+#define	CTL_HW	6
+#define	HW_NCPU	3
+
+static int32
+getncpu(void)
+{
+	uint32 mib[2];
+	uint32 out;
+	int32 ret;
+	uintptr nout;
+
+	// Fetch hw.ncpu via sysctl.
+	mib[0] = CTL_HW;
+	mib[1] = HW_NCPU;
+	nout = sizeof out;
+	out = 0;
+	ret = runtime·sysctl(mib, 2, (byte*)&out, &nout, nil, 0);
+	if(ret >= 0)
+		return out;
+	else
+		return 1;
+}
+
 // Basic spinlocks using CAS. We can improve on these later.
 static void
 lock(Lock *l)
@@ -112,6 +136,7 @@ runtime·newosproc(M *m, G *g, void *stk, void (*fn)(void))
 void
 runtime·osinit(void)
 {
+	runtime·ncpu = getncpu();
 }
 
 void