x86: efi: Set the correct link flags for the 64-bit EFI app

At present some 32-bit settings are used with the 64-bit app. Fix this by
separating out the two cases.

Be careful not to break the 64-bit payload, which needs to build a 64-bit
EFI stub with a 32-bit U-Boot.

Signed-off-by: Christian Melki <christian.melki@t2data.com>
Signed-off-by: Simon Glass <sjg@chromium.org>