Fix NXDK build for Xbox.

This disables the WASAPI, DirectSound and WinMM backends which means
you will not get any actual audio output working. An Xbox backend will
need to come later. The main purpose of this commit is to get the main
library compiling.

The main complication arises from the fact that both _WIN32 and
_MSC_VER are defined which makes miniaudio think it's using a normal
desktop Windows build. In practice it mostly works, but there's a few
things needing to be changed specifically for NXDK:

  - `fopen_s()` is not a thing with NXDK. It always uses `fopen()`.
  - There is no `_wfopen()`, nor `wcsrtombs()`, so attempting to open
    a file from a wide character string will fail.
  - There is also no `CreateFileW()`, so this code path will also
    result in an error if you attempt to open a file from a wide
    character path.
  - `CoInitialize()` is not a thing with NXDK and has therefore been
    excluded from the build.
  - `GetFileInformationByHandle()` does not exist, and neither does
    `struct stat` or `stat()`. Since the only file information miniaudio
    attempts to retrieve is the file size, I've implemented a fall back
    which uses the seek/tell/seek pattern when info retrieval is
    unavailable.
  - A fall back has been implemented for comparing wide character path
    extensions which performs a case-sensitive compare instead. This
    means that if you are using wide character paths, miniaudio will not
    detect an extension like "wav" and "WAV" as the same thing. This
    might be made more robust later if there is enough demand.

Public issue https://github.com/mackron/miniaudio/issues/1023