runtime: copy more of scheduler from Go 1.7 runtime

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