Stubbed IsSdmcWriteable to always return writeable.

While I could base this PR on #180, I found that PR to be significantly incomplete, and it would take considerable effort to add all of the required checks to the SDMC functions (which would require overriding all of them in every single archive that accessed the SD card).

To this end, I decided that for now it would probably be best to just have it always return the writeable status, and this should get retail programs a little bit farther.