runtime: add NumCPU

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5528061

src/pkg/runtime/extern.go

@@ -107,6 +107,9 @@ func (f *Func) FileLine(pc uintptr) (file string, line int) {
 // mid returns the current os thread (m) id.
 func mid() uint32
 
+// NumCPU returns the number of CPUs on the local machine.
+func NumCPU() int
+
 // Semacquire waits until *s > 0 and then atomically decrements it.
 // It is intended as a simple sleep primitive for use by the synchronization
 // library and should not be used directly.

src/pkg/runtime/runtime1.goc

@@ -8,3 +8,7 @@ package runtime
 func GOMAXPROCS(n int32) (ret int32) {
 	ret = runtime·gomaxprocsfunc(n);
 }
+
+func NumCPU() (ret int32) {
+	ret = runtime·ncpu;
+}