xcc: define __INT*_C() and __UINT*_C()

Our minimal C library makes an alias of UINT*_C() to
be __UINT*_C() and INT*_C() to __INT*_C(). However,
in XCC, these are not defined by default, so define
them ourselves.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>