RISC-V: initial parser benchmark

What

Add initial parser benchmarking tool.

Why

As we optimise the parser - we'll be able to use this to compare improvements/ensure that we are making progress.

Manually testing the MR

$ cd src/riscv
$ make riscv-sandbox

# Random data
$ KERNEL=$(mktemp)
$ head -c 16M </dev/urandom >$KERNEL

$ ./riscv-sandbox bench parser jstz/target/riscv64gc-unknown-hermit/release/jstz $KERNEL --iter 20
jstz/target/riscv64gc-unknown-hermit/release/jstz
- Bytes parsed: 16633368
- Average:      157.63ms
- Throughput:   41,980,848.07 Instructions/s
- Runs:         20
/run/user/1000/tmp.XVz3FvHtdF
- Bytes parsed: 16777216
- Average:      215.95ms
- Throughput:   31,071,846.49 Instructions/s
- Runs:         20