arm64: Implement efi_rt_arch_call

[andrew]

Summary

This is a function that calls into the EFI Runtime Services, but can
handle a fault. To support this add a handler in assembly that can
restore the kernel state on an exception and return a failure to the
caller.

Sponsored by: Arm Ltd

[andrew]

Add vm_fault_disable_pagefaults/vm_fault_enable_pagefaults around calls to the EFI runtime services