debug: Don't rely on RISCV env

That made sense when mostly people used riscv-tools, but now they get
tools from all sorts of places and most of them are suitable for the
debug tests.

Also document RISCV_TESTS_DEBUG_GCC and RISCV_TESTS_DEBUG_GDB
environment variables in the README.

The github workflows that rely on these tests don't use the Makefile,
but instead invoke gdbserver.py directly, so they're not affected by
this change.

Fixes #481