powerpc: Fix asm EMIT_BUG_ENTRY with !CONFIG_BUG

Instead of not defining it at all, this defines the macro as
being empty, thus avoiding ifdef's in call sites when CONFIG_BUG
is not set.

Also removes an extra whitespace in the existing definition.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>