[clang] [RISCV] Implement multi-lib reuse rule for RISC-V bare-metal toolchain (PR #73765)

[antoine moynault via cfe-commits]

antmox wrote:

Hello, looks like it broke some bots too:
clang-aarch64-quick https://lab.llvm.org/buildbot/#/builders/188/builds/39436
clang-armv8-quick https://lab.llvm.org/buildbot/#/builders/245/builds/18162

https://github.com/llvm/llvm-project/pull/73765