[flang] [clang] [Flang] Support -mrvv-vector-bits flag (PR #77588)

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

Thanks for addressing my comment! 

The overall logic LGTM, but please wait for somebody to review the finer RISC-V details before landing this.

https://github.com/llvm/llvm-project/pull/77588