Summary

The goal of this PR is to standardise the approach to building C++ sources in MicroPython ports that use simple Makefiles (esp8266, minimal, mimxrt, renesas-ra, samd, stm32, unix, windows with mingw). This removes a TODO that had been copied into the Makefile of all the bare metal (-nostdlib) ports over the years.

Partially resolves the issues reported in #18351.

Also:

• Adds CI coverage for the example user C modules to esp8266, nrf, mimxrt, renesas-ra & windows mingw ports, meaning all of the ports mentioned above now have CI coverage for building C++ sources. (Except minimal, which isn't built in CI as far as I can tell).
• Fixes C++ compilation on the nrf port.
• Adds a default abort() implementation to mimxrt port, copied from other ports. This allows the default cppexample module to build.
• Explains the libstdc++ dynamic allocation and standard library situation in the docs

Testing

Built with USER_C_MODULES=../../examples/usercmodule and verified the example cppexample module was functional on:

• stm32 port (BOARD=PYBD_SF2)
• samd port (BOARD=SEEED_WIO_TERMINAL)
• unix port (standard variant)

Also built but didn't have hardware to test, but verified the cppfunc symbol appeared in the map file:

• mimxrt port (BOARD=TEENSY40)
• renesas-ra port (BOARD=EK_RA6M2)
• nrf port

CI should also now build at least one board/variants from each of these ports with the usercmodule modules.

Trade-offs and Alternatives

• py/mkrules.mk is the most appropriate existing common Makefile I could find for the C++ support, but it doesn't feel 100% right in there to me. We could instead add a py/flags.mk or similar and put it there (possibly along with some other things which are the same on all Makefile ports.)