This reverts commit 00e953e.

This reverts commit 006182f.

This reverts commit 12737e2.

This reverts commit 5b70fa0.

This removes downscaling (halving-add) when multiple voices are
being mixed.  To avoid clipping, and get similar behavior to before,
set the "level" of each voice to (1/voice_count).

Slow paths that were applicable to only M0 chips were removed.

As a side effect, the internal volume representation is now 0 ..
0x8000 (inclusive), which additionally makes a level of exactly 0.5
representable.

Testing performed, on PyGamer: For all 4 data cases, for stereo and
mono, for 1 and 2 voices, play pure sign waves represented as
RawSamples and view the result on a scope and through headphones.
Also, scope the amount of time spent in background tasks.

Code size: growth of +272 bytes

Performance (time in background task when mixing 2 stereo 16-bit voices):
76us per down from 135us (once per ~2.9ms long term average)
(Decrease from 4.7% to 2.4% of all CPU time)

Introduces a way to place CircuitPython code and data into
tightly coupled memory (TCM) which is accessible by the CPU in a
single cycle. It also frees up room in the corresponding cache for
intermittent data. Loading from external flash is slow!

The data cache is also now enabled.

Adds support for the iMX RT 1021 chip. Adds three new boards:
* iMX RT 1020 EVK
* iMX RT 1060 EVK
* Teensy 4.0

Related to #2492, #2472 and #2477. Fixes #2475.

On power up the FlexRAM banks are in an unknown config so we can't
rely on the stack until after we configure FlexRAM.

Update fix (missing pragma gcc diagnostic push)

…abling it.