[llvm-jitlink] Prevent dead-stripping of test callback

`llvm_jitlink_setTestResultOverride` is used via runtime lookup by
tests, so make sure it is not dead-stripped from llvm-jitlink in release
builds.

Fixes https://github.com/llvm/llvm-project/issues/53203

Differential Revision: https://reviews.llvm.org/D117609