IntelFsp2Pkg: Update Function header to support IA32/X64.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4126



Common functions will have either 32bit or 64bit instances which
having different return code size. Function header should support both
scenarios.

Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Signed-off-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>