crt: Fix exports of crtdll.dll library

Symbols _ctype and _mbctype are DATA, like in all other crt libraries.
Symbol _onexit is read-only function. And symbols _mkgmtime, _mkgmtime32
and _mkgmtime64 are not present in crtdll.dll at all. Calling gendef on
crtdll.dll from Win NT 3.50 and Win 7 confirms it.

Signed-off-by: LIU Hao <lh_mouse@126.com>