arm64: Hook up efi_rt_arch_call in efirt.ko

Note that this requires marking efi_rt_fault as a non-preemptible
local symbol.

Co-authored-by: Jessica Clarke <jrtc27@FreeBSD.org>
Reviewed by: imp, andrew
Obtained from: CheriBSD
Fixes: d7f930b80e89 ("arm64: Implement efi_rt_arch_call")
Differential Revision: https://reviews.freebsd.org/D48715