runtime: simplify nr_cpus(), add a test for nr_jobs()

vlib/runtime/runtime_windows.c.v

@@ -2,15 +2,10 @@ module runtime
 
 import os
 
-fn nr_cpus_win() int {
+pub fn nr_cpus() int {
 	mut nr := int(C.GetCurrentProcessorNumber())
 	if nr == 0 {
 		nr = os.getenv('NUMBER_OF_PROCESSORS').int()
 	}
 	return nr
 }
-
-fn nr_cpus_nix() int {
-	eprintln('nr_cpus_nix should be callable only for nix platforms')
-	return 1
-}