[clang] [RISCV] Allow YAML file to control multilib selection (PR #98856)

[Sam Elliott via cfe-commits]

lenary wrote:

This is also a limitation if you use `@` as a prefix as suggested in the discourse thread -  GCC's `-print-multi-lib` prints out flags with the first `-` replaced with an `@`, which iirc some build systems also parse (i recall crosstool-ng, but maybe it's newlib within a crosstool-ng build).

I'm not entirely sure what the solution is here - there are two mappings going on, the yaml is optimised for the flags -> libraries translation, but not really for the "what are the (minimal) flags to produce objects to go into this library" query which `--print-multi-lib` wants.

https://github.com/llvm/llvm-project/pull/98856