crt: Define ftello64 symbol as alias to _ftelli64

mingw-w64 implementation of ftello64() and _ftelli64() functions are same.

So remove the ftello64 implementation and define ftello64 symbol as an
alias to the _ftelli64 symbol.

This change allows to use native msvcr80+ / UCRT implementation of
_ftelli64() function for the POSIX ftello64() instead of statically linking
the mingw-w64 implementation.

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