RISC-V: optionally print timestamps with debug messages

What

Add an option to the sandbox, to print 'nanosecond' timestamps to the debug log.

Why

Allows for additional tooling to pick up these timestamps, to e.g. produce accurate TPS measurements.

How

Passthrough a Console, which is used by the implementation of console_put_char, which will either do a passthrough to stdout, or - optionally - insert timings as necessary.

Manually testing the MR

$ cd src/risc_v
$ ./risc-v-sandbox rvemu \
    --input ../../tezt/tests/riscv-tests/hermit-loader \
    --initrd jstz/target/riscv64gc-unknown-hermit/release/jstz \
    --timings # try with/without this option