[libcxx] [Windows] Use the standard vsnprintf instead of _vsnprintf

In ancient Microsoft C runtimes, there might only have been
a nonstandard `_vsnprintf` instead of the standard `vsnprintf`, but
in modern versions (the only ones relevant for libc++), both
are available.

In MinGW configurations built with `__USE_MINGW_ANSI_STDIO=1` (as it
is built in CI), `vsnprintf` provides a more standards compliant
behaviour than what Microsoft's CRT provides, while `_vsnprintf` retains
the Microsoft C runtime specific quirks.

Differential Revision: https://reviews.llvm.org/D118187