powerpc: Improve 64-bit syscall entry/exit

We unconditionally hard enable interrupts. This is unnecessary as
syscalls are expected to always be called with interrupts enabled.

While at it, we add a WARN_ON if that is not the case and
CONFIG_TRACE_IRQFLAGS is enabled (we don't want to add overhead
to the fast path when this is not set though).

Thus let's remove the enabling (and associated irq tracing) from
the syscall entry path. Also on Book3S, replace a few mfmsr
instructions with loads of PACAMSR from the PACA, which should be
faster & schedule better.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>