crt: always use replacements for btowc and wctob

CRT's wctob sign-extends its return value if value of converted
single-byte character is outside of range [0,127].

This behavior makes it impossible to distinguish failure from success if
value of converted character is 255 (when sign-extended, same as EOF).

Signed-off-by: Kirill Makurin <maiddaisuki@outlook.com>