size_t must be an unsigned it. Moreover, by definition, size_t is the
return type of the "sizeof" operator, which is defined by the compiler.
Both GCC and clang define a __SIZE_TYPE__ macro to let the libc header
know about the type used by the sizeof operator.
Change-Id: Ie44f1ba69fe2b1d5ce5cd9a0e7fd8a7f10c1199d
