[llvm-branch-commits] [BOLT][test] Fix callcont-fallthru.s after #129481 (PR #135867)

[Amir Ayupov via llvm-branch-commits]

https://github.com/aaupov created https://github.com/llvm/llvm-project/pull/135867

Force the use of llvm-nm for PREAGGPLT check.

Test Plan: bin/llvm-lit -a tools/bolt/test/X86/callcont-fallthru.s