efi/runtime-wrappers: Don't duplicate setup/teardown code

Avoid duplicating the EFI arch setup and teardown routine calls numerous
times in efi_call_rts(). Instead, expand the efi_call_virt_pointer()
macro into efi_call_rts(), taking the pre and post parts out of the
switch.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>