devel/llvm-cheriot: compiler for CHERIoT ISA

Add a port of the CHERIoT compiler. The CHERIoT ISA adds CHERI to
RV32e (a microcontroller RISC-V variant). A number of products based
on CHERIoT are in the pipeline including the Sonata board from lowRISC's
Sunburst Project (https://www.sunburst-project.org).

Requested by: emaste