gcc: append PR to S to match oe-core and fix do_headerfix

This fixes the gcc-cross-inital failure, as reported here:
http://thread.gmane.org/gmane.comp.handhelds.openembedded.core/6150



Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>