efi: Move common EFI stub code from x86 arch code to common location

No code changes made, just moving functions and #define from x86 arch
directory to common location.  Code is shared using #include, similar
to how decompression code is shared among architectures.

Signed-off-by: Roy Franz <roy.franz@linaro.org>
Acked-by: Mark Salter <msalter@redhat.com>
Reviewed-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>