[RISCV] Add the zve extension according to the v1.0 spec

`zve` is the new standard vector extension to specify varying degrees of
vector support for embedding processors. The `zve` extension is related
to the `zvl` extension and other updates that are added in v1.0.

According to https://github.com/riscv-non-isa/riscv-c-api-doc/pull/21,
Clang defines macro `__riscv_v_max_elen`,  `__riscv_v_max_elen_fp` for
`zve` and it can be used by applications that uses the vector extension.

Authored by: Zakk Chen <zakk.chen@sifive.com> @khchen
Co-Authored by: Eop Chen <eop.chen@sifive.com> @eopXD

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D112408