Fix clang unit tests

[dkjer]

On Mac, clang errors out on some flags when attempting to run tests.

Description

On Mac, clang errors out on some flags when attempting to run tests:
clang: error: the clang compiler does not support '-funsigned-bitfields' [-Werror]
clang: error: optimization flag '-fno-inline-small-functions' is not supported [-Werror,-Wignored-optimization-argument]
clang: error: unsupported argument '-adhlns=.build/test_obj/eeprom_stm32/tmk_core/common/test/flash_stm32_mock.lst' to option 'Wa,'

Steps to reproduce:

• On a system using clang, run make test:debounce_sym_defer_g

This change skips including a few non-compatible compiler flags when using clang.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[zvecr]

Continuing from #12567 (comment).

The main issue this is trying to solve is clangs limited gcc compatibility. clang can be run on more than systems that report as apple, likewise gcc can work fine on x86 darwin systems. There seems to still be little movement in providing a gcc for apple Arm64.

So would still be keen to pivot the issue from "is apple" to "is clang".

[dkjer]

@zvecr Ok, updated to check for clang