Add option for link concurrency.
Add option for link concurrency.

Skia's buildbots, particularly Windows builds with LTO, are held back by the
link pool.  We'd like to play around with this number a bit.  Any problem
checking an environment variable first before doing the RAM-based calculation?

Testing: ran with and without GYP_LINK_CONCURRENCY=90, looked at build.ninja

Patch from mtklein@chromium.org.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/315343003
witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp