[clang] [llvm] [RISCV][VLS] Support RISCV VLS calling convention (PR #100346)

[Brandon Wu via llvm-commits]

4vtomat wrote:

> Is there a downside if we just recommend people `__attribute__((riscv_vls_cc(MY_LIBRARY_VLEN)))`, then put `-DMY_LIBRARY_VLEN=128` on the command-line? It's about the same complexity as -mriscv-abi-vlen, and it's much more obvious that what you're doing is fragile.

After discussed with @kito-cheng, we thought this concern makes sense for us, it would be easier to debug if we use custom defined macro rather than command line option. We can keep the default value(128 bits) but remove the option, people can use their own way to define the ABI_VLEN. Thanks @efriedma-quic for the suggestion!

https://github.com/llvm/llvm-project/pull/100346