extmod/modplatform: Expose CPU features/extensions.

[agatti]

This PR adds the ability to expose CPU-specific features/extensions to scripts when the platform module is compiled in.

Right now this is only available on RISC-V RV32, but I looked into extending it for other architectures. Some Cortex CPUs have a pair of registers called ID_PFR but no flags are defined in CMSIS. For x86/x64 accessing those flags would be OS-dependent, but still doable if you limit yourself to Linux and Windows. Finally, for Aarch64 the ID_AA64PFR registers have documented flags, but I have no Armv8 to test an eventual implementation on.

The function name is still somewhat tentative as different architectures may have different names for the same thing after all.

For RISC-V, it is not implemented as a singleton with precomputed values like other functions in the platform module because it is technically possible to disable extensions by clearing the bits in the MISA special register rather than reading them.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.42%. Comparing base (e9c898c) to head (090c922).

❗ Current head 090c922 differs from pull request most recent head 8390cc7

Please upload reports for the commit 8390cc7 to get more accurate results.

Files	Patch %	Lines
extmod/modplatform.c	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #15268      +/-   ##
==========================================
- Coverage   98.42%   98.42%   -0.01%     
==========================================
  Files         161      161              
  Lines       21204    21205       +1     
==========================================
  Hits        20870    20870              
- Misses        334      335       +1     

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[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:  +184 +0.022% standard[incl +64(data)]
      stm32:   +28 +0.007% PYBV10
     mimxrt:   +48 +0.013% TEENSY40
        rp2:   +48 +0.005% RPI_PICO_W
       samd:   +40 +0.015% ADAFRUIT_ITSYBITSY_M4_EXPRESS

[dpgeorge]

The platform module is a CPython standard module, so we can't add custom extensions to it.

Existing CPython functions such as platform.machine() and platform.processor() may be good places to put this info.

Otherwise, we already do have sys.implementation._machine which could also store this info. Eg on stm32 it stores the full MCU name, such as:

>>> import sys
>>> sys.implementation
(name='micropython', version=(1, 24, 0, 'preview'), _machine='PYBD-SF6W with STM32F767IIK', _mpy=8966)

[agatti]

Understood. I moved it to platform.processor(), and it now returns an empty string if it's not running on a RV32 processor. This should be in line with CPython's behaviour.