rtloader: Use execinfo only on glibc

[at-wat]

What does this PR do?

Use execinfo only on glibc.
Functions in execinfo.h are GNU extensions and not available on other C libraries like musl.

Motivation

We used to use libexecinfo package (A quick-n-dirty BSD licensed clone of the GNU libc backtrace facility.) of Alpine Linux to build datadog-agent on Alpine, but it has been removed since Alpine 3.17.
This PR allow to build datadog-agent on Alpine Linux and other non-glibc environments.

Additional Notes

A docker image based on Alpine Linux (musl libc) with this patch is available at
https://github.com/seqsense/datadog-agent-alpine/pkgs/container/datadog-agent/69345371?tag=7.41.1-alpine.
I tested with out DD_API_KEY to confirm logs and python based metrics integrations work.

Possible Drawbacks / Trade-offs

No change to the official builds.

Describe how to test/QA your changes

Build result will have no change to the official docker images and the binaries for Ubuntu and other glibc based linux distros.

To ensure that stacktraces are still printed for glibc-based distros, run the following steps on either the docker image or a host install on ubuntu.

1. Add a python check that segfaults, an example has been provided below
2. Configure an instance of this check to run
3. Enable the config option: c_stacktrace_collection: true
4. Run the agent, once the segfaulting check runs, the agent should print out HANDLER CAUGHT signal Error: signal 11 C-LAND STACKTRACE: followed by a stacktrace.

from datadog_checks.base import AgentCheck
import ctypes;

class HelloCheck(AgentCheck):
    def check(self, instance):
        # This tries to read a word from nil, leading to segv
        # from: https://bugs.python.org/issue1215#msg143236
        ctypes.string_at(0)

Reviewer's Checklist

• If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• Use the major_change label if your change either has a major impact on the code base, is impacting multiple teams or is changing important well-established internals of the Agent. This label will be use during QA to make sure each team pay extra attention to the changed behavior. For any customer facing change use a releasenote.
• A release note has been added or the changelog/no-changelog label has been applied.
• Changed code has automated tests for its functionality.
• Adequate QA/testing plan information is provided if the qa/skip-qa label is not applied.
• At least one team/.. label has been applied, indicating the team(s) that should QA this change.
• If applicable, docs team has been notified or an issue has been opened on the documentation repo.
• If applicable, the need-change/operator and need-change/helm labels have been applied.
• If applicable, the k8s/<min-version> label, indicating the lowest Kubernetes version compatible with this feature.
• If applicable, the config template has been updated.

[scottopell]

changes needed for bsd

[scottopell]

We support BSD backtraces as well (at least freebsd), so this is a bit restrictive I think.

Maybe a better approach would be to add a set HAS_BACKTRACE_LIB to the cmake section here for libexec / backtrace and replace this with #ifdef HAS_BACKTRACE_LIB

Leaving this here for future reference/testing - could mimic llvm's structure to potentially support more BSDs: https://reviews.llvm.org/D27143#differential-review-stage
But out-of-scope for this change

[at-wat]

Addressed by 30518b6 and confirmed it works on Alpine at seqsense/datadog-agent-alpine#82

[scottopell]

This is great, but we also need this a few lines below in the ## Backtrace section for FreeBSD as well. According to the freebsd man pages, the Backtrace package also provides execinfo.h

[scottopell]

I added a QA plan to your comment, feel free to proof-read.

[at-wat]

Is there anything I need to do for merging?

[scottopell]

@at-wat Apologies, this one seems to have slipped through the cracks. I'm going to try to get our full CI pipeline running on this and once that goes green we're good to merge.
Nothing else is needed from you at this time, and thanks again for the contribution!

[scottopell]

We're in a merge freeze at the moment, but whenever that lifts I'll override the CI checks here and merge this. I rebased these changes onto the latest main and CI is all green b7d7bb1