crt: Provide emulation of _ctime64 and _wctime64 functions

Functions _ctime64 and _wctime64 are available since msvcr70.dll. For older
msvcrt versions provide mingw-w64 emulation via asctime() and _wasctime()
functions and then via _localtime64() function which is now available in
all CRT import libraries (either as native symbols or as mingw-w64 emulation).

Signed-off-by: Martin Storsjö <martin@martin.st>