powerpc: Refactor BUG/WARN macros

BUG(), WARN() and friends are using a similar inline assembly to
implement various traps with various flags.

Lets refactor via a new BUG_ENTRY() macro.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c19a82b37677ace0eebb0dc8c2120373c29c8dd1.1566219503.git.christophe.leroy@c-s.fr