Fix: Rename RISC-V machine scratch CSR

[ALTracer]

Detailed description

• This is a small typo fix.
• The existing problem is tscratch description for gdb target.xml of MSCRATCH CSR.
• The PR solves it by renaming to mscratch.

Not tested on anything, because I don't have a gd32vf1/sipeed longan nano, and ESP32-C3 needs burning OTP eFuses for external JTAG (USJ is not supported by BMDA). Unsure what uses this.

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]

This LGTM - merging. Thanks for the contribution!