Bump RV32E version to 2.0

RV32E was recently [ratified](https://github.com/riscv/riscv-isa-manual/commit/afd613691cb89ccd7584206e8a6d1866fe77ec88) so we should update the version as our MC-layer support is complete.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D144384