Add support for the fpu_kern(9) KPI on arm64. It hooks into the existing
VFP code to store the old context, with lazy loading of the new context
when needed.

FPU_KERN_NOCTX is missing as this is unused in the crypto code this has
been tested with, and I am unsure on the requirements of the UEFI
Runtime Services.

Reviewed by: kib
Obtained from: ABT Systeems Ltd
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8276