[efi] Do not rely on ProcessorBind.h when building host binaries

[efi] Do not rely on ProcessorBind.h when building host binaries

We cannot rely on the EDK2 ProcessorBind.h headers when compiling a
binary for execution on the build host itself (e.g. elf2efi), since
the host's CPU architecture may not even be supported by EDK2.

Fix by skipping ProcessorBind.h when building a host binary, and
defining the bare minimum required to allow other EDK2 headers to
compile cleanly.

Reported-by: Michal Suchánek <msuchanek@suse.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>