Fix: Report malloc failures in target_description of cortexm, cortexar

[ALTracer]

Detailed description

• No new features, it's more of a fix.
• The existing problem is unchecked malloc in regs_description.
• This PR adds the check and logs on heap exhaustion.

Discovered during testing around July 14. Triggerable by PR1830 (stack front) plus no PR1529 (traceswoasync on heap) on stlink/swlink. (hits even harder with ENABLE_RTT=1). Otherwise the symptom on attach is rather cryptic, GDB gets no (or malformed) register description and defaults to wrong architecture and register numbers (use RSP logging).
May also be beneficial not just to ENABLE_DEBUG=1 builds, but also out-of-tree projects which have RTOS and log channels but a small heap for userspace tasks.

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• I've read the Code of Conduct
• I've read the guidelines for contributing to this repository
• It builds for hardware native (see Building the firmware)
• It builds as BMDA (see Building the BMDA)
• I've tested it to the best of my ability
• My commit messages provide a useful short description of what the commits do

Closing issues