tests: Test pac-ret flag merging on clang with LTO

[mrkajetanp]

Extend the test for pac-ret with clang and LTO by checking that different branch protection flags are preserved after the LTO step. There was an issue in older LLVM versions that was causing this to behave incorrectly.

[rustbot]

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[rustbot]

This PR modifies tests/run-make/. If this PR is trying to port a Makefile
run-make test to use rmake.rs, please update the
run-make port tracking issue
so we can track our progress. You can either modify the tracking issue
directly, or you can comment on the tracking issue and link this PR.

cc @jieyouxu