[LoongArch] Support 'generic' as a valid CPU name

As the LoongArch port is largely modeled after RISCV it has the same
behavior of not accepting `generic` as a CPU name. For better
compatibility with consumers of LLVM (e.g. mesa) follow D121149's suit
and treat `generic` the same as an empty CPU name.

Differential Revision: https://reviews.llvm.org/D134412