RISC-V: Compile jstz kernel to Hermit target

Context

This MR initialises a crate which builds a forked version of the jstz kernel to the Hermit target.

Manually testing the MR

$ make -f kernels.mk jstz

Loading with qemu should succeed. Execution fails on the first unimplemented host function.

$ qemu-system-riscv64 -display none -serial stdio -kernel path/to/hermit-loader-riscv64  -machine virt -cpu rv64 -initrd src/risc_v/jstz/target/riscv64gc-unknown-hermit/release/jstz -smp 1 -m 256M

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR