GDB: enable extensions for RISC-V

[mkobetic]

This PR moves bulk of the GDB extensions into core/dev to maximize shared code, and then arm/dev and rv/dev have architecture specific bits.

• amforth.gdb is now split between arch/dev/amforth.gdb and core/dev/amforth-core.gdb that is imported by the former
• gdb_arch.py is now the architecture specific python bits, mostly about the differences between the register sets.

Note that the Python API requires a reasonably recent version of GDB (apparently Arduino WCH toolchain has some fairly old GDB that won't work with these extensions).

Otherwise everything should be working for RISC-V including backtrace unwinding.