LinuxKPI: Add explicit software context to FPU sections

Amdgpu driver does a lot of memory allocations in FPU-protected sections
of code for certain display cores, e.g. for DCN30. This does not work
currently on FreeBSD as its malloc function can not be run within a
critical section. Allocate memory for FPU context to overcome such
restriction.

Sponsored by: Serenity Cyber Security, LLC
Reviewed by: manu (previous version), markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42822