ppc: Improve a few more helper flags

Mostly turn "store" type of helpers into TCG_CALL_NO_WG because
they can take exceptions. Also fixup_thrm doesn't read nor write
the tracked environment.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>