Fix logic for enabling cloned functions

[brad0]

Summary

A fix went in to fix building with Musl but the logic was flawed.
Instead of only checking __GLIBC__ on Linux or Hurd it checks this
everywhere and __GLIBC__ has no relevance outside of Linux or Hurd.

Check for __OpenBSD__ which does not have ifunc support.

Check for PPC also __GLIBC__.

From FreeBSD/ppc64..

iop_profile.c:1273:6: error: 'target_clones' attribute needs GLIBC (2.23 and newer) that exports hardware capability bits

Checklist

• I have read CONTRIBUTING.md and the coding style.
• I have not merged master into the topic branch.
• The pull request is one logical change, and every commit compiles on its own.
• I ran the relevant tests: unit tests, src/tests/integration/ where the pixelpipe is touched, or darktable-cli as a headless smoke test.
• New user-visible strings use _(), new preferences are registered in data/darktableconfig.xml.in.
• A RELEASE_NOTES.md entry was added (only needed if fixing an issue in a release). Do not reference GitHub issues.

Test instructions

Build on OpenBSD and it fails to build due to ifunc usage.

Build on FreeBSD/ppc64 (without the bogus __GLIBC__ check) and it fails to build with the error above.