[bazel] Add cc_rules-based lowRISC toolchain

This uses the new rules-based toolchain feature of `rules_cc` to define
the lowRISC toolchain with the same flags that the CRT toolchain had.

There are two differences:

1. The CRT toolchain does not have the `guards` feature enabled by
   default, we enable it in `.bazelrc`. This new one is enabled
   automatically.

2. The CRT toolchain has `-Wpedantic` enabled by default, but we disable
   it in `.bazelrc`. This new one has it disabled by default.

Signed-off-by: James Wainwright <james.wainwright@lowrisc.org>

Author: jwnrt