[clang] [clang-tools-extra] [llvm] [RISCV] Clang flags for controlling zilsd alignment (PR #181439)

[Sam Elliott via cfe-commits]

lenary wrote:

The Toolchain conventions PR has been approved and the implementation has landed in GCC, so I'd like to get this merged soon :) 

https://github.com/llvm/llvm-project/pull/181439