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

llvm based toolchains need the same fix as exists for xcc that was
introduced with commit:

commit 5ef8db8a
Author: Daniel Leung <daniel.leung@intel.com>
Date:   Wed Feb 3 14:23:51 2021 -0800

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

Since llvm based toolchains don't define these macros either.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>