target-ppc: Introduce tm_enabled Bit to CPU State

Add a bit (tm_enabled) to CPU state that mirrors the MSR[TM] bit.
This is analogous to the other "available" bits in the MSR (FP,
VSX, etc.).

NOTE: Since MSR[TM] occupies big-endian bit 31, the code is wrapped
with a PPC64 bit check.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
