gcc: use <pkg>_EXCLUDES, not <pkg>_TAR_EXCLUDES

As reported by Steven Noonan, the variable recently introduced in the
package infrastructure to exclude certain parts of an archive from
being extracted is <pkg>_EXCLUDES, not <pkg>_TAR_EXCLUDES. However,
the gcc code was incorrectly using <pkg>_TAR_EXCLUDES. This commit
fixes that.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reported-by: Steven Noonan <steven@uplinklabs.net>