kernel - Fix signal FP save/restore issues when AVX is enabled
* The kernel was not saving/restoring the full FP context when entering into
or returning from a signal, leading to corrupt FP registers even when
AVX is not used, when AVX is enabled in the kernel.
ANY SIGNAL COULD CORRUPT THE FP STATE.
* Fixed by adjusting the on-user-stack fpsave area sizes and operation.
* This unfortunately changes a number of user visible structures.
ucontext_t, mcontext_t, sigcontext, sigframe.
It is POSSIBLE that most userland use cases will be unaffected, but I'm
not holding my breath.
Major-Sleuthing-by: ftigeot
Testing-by: ftigeot, dillon