C++20 volatile warnings

[GFoxPM]

Lots of warnings: compound assignment with 'volatile'-qualified left operand is deprecated [-Wvolatile]
Fixes like:
/src/rp2_common/cmsis/stub/CMSIS/Core/Include/mpu_armv7.h#L210

-  MPU->CTRL  &= ~MPU_CTRL_ENABLE_Msk;
+  MPU->CTRL  = MPU->CTRL & ~MPU_CTRL_ENABLE_Msk;

[kilograham]

@will-v-pi is this fixed with latest CMSIS headers?

[will-v-pi]

No, although I think it's being looked into - see ARM-software/CMSIS_5#1544 and ARM-software/CMSIS_6#121

These CMSIS stubs are taken directly from the upstream CMSIS repo, so we won't be modifying them in this repo. Once they're fixed there we'll update our headers here