target/ppc: Fix SPE unavailable exception triggering

target/ppc: Fix SPE unavailable exception triggering

When emulating certain floating point instructions or vector instructions on
PowerPC machines, QEMU did not properly generate the SPE/Embedded Floating-
Point Unavailable interrupt. See the buglink further below for references to
the relevant NXP documentation.

This patch fixes the behavior of some evfs* instructions that were
incorrectly emitting the interrupt.

More importantly, this patch fixes the behavior of several efd* and ev*
instructions that were not generating the interrupt. Triggering the
interrupt for these instructions fixes lazy FPU/vector context switching on
some operating systems like Linux.

Without this patch, the result of some double-precision arithmetic could be
corrupted due to the lack of proper saving and restoring of the upper
32-bit part of the general-purpose registers.

Buglink: https://bugs.launchpad.net/qemu/+bug/1888918
Buglink: https://bugs.launchpad.net/qemu/+bug/1611394
Signed-off-by: Matthieu Bucchianeri <matthieu.bucchianeri@leostella.com>
Message-Id: <20200727175553.32276-1-matthieu.bucchianeri@leostella.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>