[PROF-9926] Fix rpath for linking to libdatadog when loading from extension dir (cherry-pick from 1.x-stable)

[ivoanjo]

What does this PR do?

This is a cherry-pick of #3683 which was first applied to the 1.x-stable branch. See that PR for details.

The diff is slightly different in the spec file since 1.x-stable needed a few tweaks to support older Rubies, which 2.x doesn't need. Everything else is the same.

Motivation:

Fix the rpath issue.

Additional Notes:

N/A

How to test the change?

See #3683 && and additional integration test for this was added on https://github.com/DataDog/prof-correctness/pull/39 .

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 98.11%. Comparing base (bd08222) to head (3979b55).

Files	Patch %	Lines
...datadog/profiling/native_extension_helpers_spec.rb	95.65%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3706 +/- ## ========================================== - Coverage 98.11% 98.11% -0.01% ========================================== Files 1225 1225 Lines 72806 72836 +30 Branches 3482 3485 +3 ========================================== + Hits 71431 71460 +29 - Misses 1375 1376 +1 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.