efi.h: Do not use config options

Currently efi.h determines a few bits of its environment according to
config options. This falls apart with the efi stub support which may
result in efi.h getting pulled into the stub as well as real U-Boot
code. In that case, one may be 32bit while the other one is 64bit.

This patch changes the conditionals to use compiler provided defines
instead. That way we always adhere to the build environment we're in
and the definitions adjust automatically.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: added some comments to describe the __x86_64__ check]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>