introduce svp64 prefixing

introduce svp64 prefixing

This (very incomplete) patch introduces svp64 prefixing support in the
GNU assembler.

The idea is to mark opcodes (in .../ppc-opc.c) that can be prefixed
with XT2 or XT3, and svp64 opcodes that aren't prefixed (setvl) with
OPC.

???  I'm not sure XT2 and XT3 are enough, though.  We might need to
know the RM-* encoding to be able to parse the prefixed opcode.
That's where I got stuck a few weeks ago.