crt: Add fallback _vscprintf() implementation

Original MSVC 6.0 msvcrt.dll library does not provide _vscprintf()
function. MinGW-w64 __ms_snprintf() and __ms_vsnprintf() implementations
call _vscprintf() function. So include fallback _vscprintf() implementation
into MinGW-w64 crt code to allow applications to use snprintf() with
original MSVC 6.0 msvcrt.dll library.

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