Use nested interrupts (NVIC) in play_sd_raw.cpp

[nsasch]

This copies over the changes that were made in 6369a6a to fix the same issue for raw audio files. The issues are also discussed in this forum post: https://forum.pjrc.com/index.php?threads/teensy-4-0-first-beta-test.54711/page-127#post-207700

Before this change, playing a second raw file while one was already playing would result in inconsistent behaviour. I observed it doing one of three things:

• it would work as desired ~30% of the time
• freeze all code execution ~50% of the time
• freeze for 1-5 seconds, not play the second file, and return, resuming code immediately after the call to AudioPlaySdRaw.play