gyp: use all processors instead of hard-coded 8.

Makes no difference for me but might help others because gyp can easily
keep lots of processors busy.

BUG=362075

Originally landed at r1896 and backed out at r1911. This should be safe
to land after r1913.

Patch by Daniel Bratell <bratell@opera.com>.

R=torne@chromium.org

Review URL: https://codereview.chromium.org/267023005

git-svn-id: http://gyp.googlecode.com/svn/trunk@1928 78cadc50-ecff-11dd-a971-7dbc132099af