[lld] Add `REQUIRES: riscv` to test added in 151639 to skip the test when riscv is not built. (PR #152858)

[via llvm-commits]

dyung wrote:

This should fix the following buildbot failures:
- https://lab.llvm.org/buildbot/#/builders/144/builds/32422
- https://lab.llvm.org/buildbot/#/builders/46/builds/21430
- https://lab.llvm.org/buildbot/#/builders/190/builds/25128
- https://lab.llvm.org/buildbot/#/builders/23/builds/12957

https://github.com/llvm/llvm-project/pull/152858