[lld] [llvm] [RISCV] Change RISCVABI::computeTargetABI() to return Expected<ABI> (PR #213410)

[Alexander Richardson via llvm-commits]

arichardson wrote:

https://github.com/llvm/llvm-project/pull/223606 should be a hopefully minimal fix for this. Missing propagation of target features with LTO has been a long running issue, and there is probably something cleaner that can be done but I think for now this restores the existing behaviour while keeping the new diagnostics for llvm-mc.

https://github.com/llvm/llvm-project/pull/213410