Implement multi-core builds.

This is done using a cl.exe compatability wrapper around
gcc that supports multiple sources files and builds them
in parallel.

BUG=

Review URL: https://codereview.chromium.org/11044024

git-svn-id: https://nativeclient-sdk.googlecode.com/svn/trunk/src@1430 050acbb0-2703-11df-ab0a-9f3f633ae91d
10 files changed