[RISCV] Bump I, F, D, and A extension versions to 20191214 spec version

[RISCV] Bump I, F, D, and A extension versions to 20191214 spec version

New versions I2.1, F2.2, D2.2 A2.1

Make F and Zfinx imply Zicsr.
Make G imply Zifencei.

This should have no impact to generated code. We have no plans to require Zicsr/Zifencei extension to be explicitly enabled to use Zicsr/Zifencei instructions in assembly.

See https://reviews.llvm.org/D147183 for documentation regarding what version specification we implement.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D147179