clang: move BPN assignment from common.inc

clang: move BPN assignment from common.inc

There's a BPN assignment in common.inc which means all recipes need to
either be called clang, or set BPN themselves.

Move the assignment to the clang recipes. For now I'm leaving the
existing BPN assignments in the other recipes, in case there are complex
multilib-related reasons to retain them.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>