efi/libstub: unify EFI call wrappers for non-x86

We have wrappers around EFI calls so that x86 can define special
versions for mixed mode, while all other architectures can use the
same simple definition that just issues the call directly.
In preparation for the arrival of yet another architecture that doesn't
need anything special here (RISC-V), let's move the default definition
into a shared header.

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