[CORE] Make size_t and ssize_t compliant with stddef.h

We have stddef.h included at number of places (mostly lwIP) which
has conflicting typedef for size_t and ssize_t hence to avoid
issues in compilation we make our version of size_t and ssize_t
compliant with GCC stddef.h

Signed-off-by: Anup Patel <anup@brainfault.org>