[RISCV] Add Zce extension.

According to the spec, Zce is an alias for Zca, Zcb, Zcmp, and Zcmt.
If F is enabled on RV32 it also includes Zcf.

This patch adds the Zce and the implication rule which unfortunately
requires custom handling for adding Zcf.

I've also made all the Zc* extensions imply Zca.

I've also added an error for Zcf without RV32.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D153742