[lld] [lld][RISCV] Add break to nested switch in `mergeAtomic` (PR #99762)

[Brandon Wu via llvm-commits]

4vtomat wrote:

> Right, but I don’t recall seeing that with a pretty recent clang, and I want to know if you’re using gcc or another compiler so I know if I should see if we should fix that in clang.

Actually it's during compiling the `lld` itself lol~

https://github.com/llvm/llvm-project/pull/99762