Add support for Linux kernels >=3.0-rc1 in Python related build files by:

 - replacing occurences of sys.platform == 'linux2' with
    sys.platform.startswith('linux')
 - congregating occurences of sys.platform in ('linux[X]', 'linuxY', ...)
    to sys.platform.startswith('linux')
  - adding the key 'linux3' to all relevant lookup dicts

BUG=85845
TEST=Try building chromium on Linux >=3.0-r1

Review URL: http://codereview.chromium.org/7172016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@89354 0039d316-1c4b-4281-b951-d872f2087c98