bazel rules for a riscv64 bare metal GCC compiler

The repository https://github.com/filmil/bazel_rules_gcc_riscv64_baremetal contains bazel rules that can be used for a hermetic bare metal GCC cross-compiler for a 64-bit RISC-V machine, running in M-mode.

It is based on the toolchain for the muntjac processor. Based on the lowRISC toolchains repo at: https://github.com/lowRISC/lowrisc-toolchains, and some custom patches.