runtime: implement NumCPU for -scheduler=threads

For the threads scheduler, it makes sense to have NumCPU available.
For all other schedulers, the number of available CPUs is practically
limited to one by the scheduler (even though the system might have more
CPUs).

src/runtime/debug.go

@@ -1,14 +1,5 @@
 package runtime
 
-// NumCPU returns the number of logical CPUs usable by the current process.
-//
-// The set of available CPUs is checked by querying the operating system
-// at process startup. Changes to operating system CPU allocation after
-// process startup are not reflected.
-func NumCPU() int {
-	return 1
-}
-
 // Stub for NumCgoCall, does not return the real value
 func NumCgoCall() int {
 	return 0