um: add back support for FXSAVE registers

It was reported that qemu may not enable the XSTATE CPU extension, which
is a requirement after commit 3f17fed2 ("um: switch to regset API
and depend on XSTATE"). Add a fallback to use FXSAVE (FP registers on
x86_64 and XFP on i386) which is just a shorter version of the same
data. The only difference is that the XSTATE magic should not be set in
the signal frame.

Note that this still drops support for the older i386 FP register layout
as supporting this would require more backward compatibility to build a
correct signal frame.

Fixes: 3f17fed2 ("um: switch to regset API and depend on XSTATE")
Reported-by: SeongJae Park <sj@kernel.org>
Closes: https://lore.kernel.org/r/20241203070218.240797-1-sj@kernel.org


Tested-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20241204074827.1582917-1-benjamin@sipsolutions.net


Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>