Fix: remove last VLA in msp432p4.c

[ALTracer]

Detailed description

• Not a new feature.
• The existing problem is VLA/alloca left over from PR1606 refactoring, specifically v1.9.0-1188-g f94ed09.
• This PR completes the replacement (to arrays with known size of 20+33 matching Cortex-M4F).

Found using ag alloca\\\( src/, also matches git grep "alloca(" origin/main -- src/target/*.c. Related: v1.10.0-244-g c0a8672 of PR1661.
Not tested.

Your checklist for this pull request

• 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

[dragonmux]

LGTM, merging. Thank you for the contribution!