fix signedness of UINT32_MAX and UINT64_MAX at the preprocessor level

fix signedness of UINT32_MAX and UINT64_MAX at the preprocessor level

per the rules for hexadecimal integer constants, the previous
definitions were correctly treated as having unsigned type except
possibly when used in preprocessor conditionals, where all artithmetic
takes place as intmax_t or uintmax_t. the explicit 'u' suffix ensures
that they are treated as unsigned in all contexts.