Henning Schild authored Sep 14, 2018 and Jan Kiszka committed Sep 17, 2018

Upstream 4.14 switched to purely eager fpu switching. That was
backported to 4.4 and 4.9. This commit makes cobalt able to deal whith
the changed kernel behaviour.
This commit takes care of 4.9 to begin with.

Introduce IPIPE_X86_FPU_EAGER to switch between the new and the old
implementations. The new implementation is much simpler than the old
one. We basically only deal with the odd case where Xenomai preempts
Linux in a kernel-fpu context.
In a regular Linux that can never happen and if it happens we need to
make sure to get things into a consistent state. We have to make
"current" as not owning the fpu anymore and allow others to use
in-kernel fpu (kernel_fpu_enable). __switch_to from Linux will do the
rest.

Signed-off-by: Henning Schild <henning.schild@siemens.com>
[Jan: converted C++-style comments]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>