[PowerPC] Support branch mnemonics with implied CR0

The extended branch mnemonics are supposed to use an implied CR0
if there is no explicit condition register specified.  This patch
adds extra variants of the mnemonics to this effect.

Problem reported by Joerg Sonnenberger.
[PowerPC] Support extended sc mnemonic

A plain "sc" without argument is supposed to be treated like "sc 0"
by the assembler.  This patch adds a corresponding alias.

Problem reported by Joerg Sonnenberger.