crt: Move the msvcrt snprintf/vsnprintf aliases into separate object files

This avoids duplicate definitions, if something refers to e.g.
__ms_vsnprintf, while the inline vsnprintf also has been instantiated
in C++ mode. (In C++ mode, the inline vsnprintf function isn't marked
static.)

This should fix https://github.com/msys2/MINGW-packages/issues/6271.

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