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

`zvl` is the new standard vector extension that specifies the minimum vector length of the vector extension.
The `zvl` extension is related to the `zve` 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_min_vlen` for `zvl` and it can be used for applications that uses the vector extension.
LLVM checks whether the option `riscv-v-vector-bits-min` (if specified) matches the `zvl*` extension specified.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D108694

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