AVX register corruption from signal delivery

[Barret Rhoden]

Hi -

The Go Team found a bug[1] where the AVX registers can get corrupted 
during signal delivery.  They were able to bisect it to commits related 
to the "x86: load FPU registers on return to userland" patchset[2].

The bug requires the kernel to be built with GCC 9 to trigger.  In 
particular, arch/x86/kernel/fpu/signal.c needs to be built with GCC 9.

Thanks,

Barret

[1] https://bugzilla.kernel.org/show_bug.cgi?id=205663
[2] 
https://lore.kernel.org/kvm/20190403164156.19645-1-bigeasy@xxxxxxxxxxxxx/