i#3544 RV64: Port sample.memtrace_simple to RISC-V

[ksco]

Implements decode_opcode_name(), which is needed by the newly enabled test. Also limits -max_bb_instrs further to 8 on RISC-V for its heavyweight instrumentation.

This is yet another test that does not work on QEMU, thus we cannot enable it in CI.

Issue: #3544

[ksco]

Running ctest -j4 on a real machine gives:

49% tests passed, 110 tests failed out of 216

That is only about half of the tests we enabled for compiling, still a long way to go.