Expose teensy4-panic's SOS

[mciantyre]

Allow a user to implement their own panic handler that can blink S.O.S. Disable the teensy4-panic's default features, then implement your own custom panic handler.

[mciantyre]

bors r+

[bors[bot]]

Build succeeded:

• clippy
• doc
• doclinks
• features
• features (rtic)
• features (rtic,rt)
• features (systick)
• features (systick,rt)
• features (usb-logging)
• features (usb-logging,rt)
• format
• precompiled
• template
• test
• xplat (macos-latest)
• xplat (ubuntu-latest)
• xplat (windows-latest)