[libc] Use function overloads to make string parsing code more generic. (#167417)
ctype_utils/wctype_utils were chaged in
120689e46679c6db37cd9e839ec0721e80a22d4f and
e7f7973899f76773ae6e9a6b1e8c7e9f9cc5cb56, respectively to operate on
char/wchar_t. Now we can switch to the overloaded names (e.g. have noth
`isspace(char` and `isspace(wchar_t)`) to simplify the templatized
strtointeger implementation from
315dfe5865962d8a3d60e21d1fffce5214fe54ef and make it easier to
potentially add templatized strtofloat implementation.