kboot: aarch64 trampoline implementation

Update exec.c (copyied from efi/loader/arch/arm64/exec.c) to allow
execution of aarch64 kernels. This includes a new trampoline code that
handles copying the UEFI memory map, if available from the Linux FDT
provided PA. This is a complete implementation now, able to boot from
the LinuxBoot environment on an aarch64 server that only offers
LinuxBoot (though a workaround for the gicv3 inability to re-init is not
yet in FreeBSD). Many 'fit and finish' issues will be addressed in
subsequent commits.

Sponsored by: Netflix
Reviewed by: tsoome, kevans, andrew
Differential Revision: https://reviews.freebsd.org/D38258