[release-branch.go1.9] runtime: fix gctrace STW CPU time and CPU fraction
    
    The CPU time reported in the gctrace for STW phases is simply
    work.stwprocs times the wall-clock duration of these phases. However,
    work.stwprocs is set to gcprocs(), which is wrong for multiple
    reasons:
    
    1. gcprocs is intended to limit the number of Ms used for mark
       termination based on how well the garbage collector actually
       scales, but the gctrace wants to report how much CPU time is being
       stolen from the application. During STW, that's *all* of the CPU,
       regardless of how many the garbage collector can actually use.
    
    2. gcprocs assumes it's being called during STW, so it limits its
       result to sched.nmidle+1. However, we're not calling it during STW,
       so sched.nmidle is typically quite small, even if GOMAXPROCS is
       quite large.
    
    Fix this by setting work.stwprocs to min(ncpu, GOMAXPROCS). This also
    fixes the overall GC CPU fraction, which is based on the computed CPU
    times.
    
    Fixes #22725.
    
    Change-Id: I64b5ce87e28dbec6870aa068ce7aecdd28c058d1
    Reviewed-on: https://go-review.googlesource.com/77710
    Run-TryBot: Austin Clements <austin@google.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Rick Hudson <rlh@golang.org>
    Reviewed-on: https://go-review.googlesource.com/88321
    Run-TryBot: Andrew Bonventre <andybons@golang.org>
    Reviewed-by: Austin Clements <austin@google.com>