Add FreeBSD as a supported platform.

FreeBSD calls GNU make 'gmake', so use a variable to call 'make' instead
of hardcoding the name.

TEST=	run 'tools/clang/scripts/update.sh --force-local-build
--without-android' on a FreeBSD system.

Maybe let the script exclude android on FreeBSD as is done for Darwin?

BUG=

Review URL: https://codereview.chromium.org/148963004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248089 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248089