powerpc: Don't allow the use of EMIT_BUG_ENTRY with BUGFLAG_WARNING

Warnings in assembly must use EMIT_WARN_ENTRY in order to generate
the necessary entry in exception table.

Check in EMIT_BUG_ENTRY that flags don't include BUGFLAG_WARNING.

This change avoids problems like the one fixed by
commit fd1eaaaa

 ("powerpc/64s: Use EMIT_WARN_ENTRY for SRR debug
warnings").

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ddcb422102a37eb45f57694c7ef0ec6187964dff.1644742951.git.christophe.leroy@csgroup.eu