Don't cap num_cores at 32 on 32-bit Windows.

See here for a similar change in Ninja.  Wanna bet they just got Z840s too?
https://github.com/martine/ninja/commit/8c8834acffdc0da0d94119725929acc712c9dfad

BUG=skia:

Review URL: https://codereview.chromium.org/1213063011
1 file changed