No functional changes intended.

This updates the test case to use llvm-nm instead of the system nm.
This fixes an instance over at the experimental build of rustc with HEAD LLVM:
https://buildkite.com/llvm-project/rust-llvm-integrate-prototype/builds/8380#ef6f41b5-8595-49a6-be37-0eff80e0ccb5
It is related to rust-lang#94001.

The issue is that this test uses the system nm, which may not be recent
enough to understand the update to uwtable. This replaces the test to
use the llvm-nm that should be recent enough (consistent with the LLVM
sources we use to build rustc).