runtime: copy more of scheduler from Go 1.7 runtime

This started by moving procresize from C to Go so that we can pass the
    right type to the memory allocator when allocating a p, which forced
    the gomaxprocs variable to move from C to Go, and everything else
    followed from that.
    
    Reviewed-on: https://go-review.googlesource.com/34916

From-SVN: r244236

libgo/go/runtime/debug.go

@@ -14,7 +14,25 @@ import (
 // change the current setting.
 // The number of logical CPUs on the local machine can be queried with NumCPU.
 // This call will go away when the scheduler improves.
-func GOMAXPROCS(n int) int
+func GOMAXPROCS(n int) int {
+	if n > _MaxGomaxprocs {
+		n = _MaxGomaxprocs
+	}
+	lock(&sched.lock)
+	ret := int(gomaxprocs)
+	unlock(&sched.lock)
+	if n <= 0 || n == ret {
+		return ret
+	}
+
+	stopTheWorld("GOMAXPROCS")
+
+	// newprocs will be processed by startTheWorld
+	newprocs = int32(n)
+
+	startTheWorld()
+	return ret
+}
 
 // NumCPU returns the number of logical CPUs usable by the current process.
 //