arm64: set FPEN if we're stuck with HCR_EL2.E2H

On Apple Silicon systems, E2H can't actually be cleared; we're stuck
with it. Check it again when we're setting up CPTR_EL2 and set FPEN
appropriately to avoid later trapping to EL2 on writes to SIMD
registers.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D38819