For coverage builds, we try to link against the `gcov` library (specific to the environment). But as macOS doesn't have this library and thus doesn't have the coverage tools to generate reports, the coverage builds on that platform were failing on linking.
We actually don't need to explicitly force this linking, as the `CodeCoverage` file already has correct detection logic (currently on lines 177-193), which is invoked when the `--coverage` flag is provided:
* AppleClang: Uses `xcrun -f llvm-cov` to set `GCOV_TOOL="llvm-cov gcov"`.
* Clang: Finds `llvm-cov` to set `GCOV_TOOL="llvm-cov gcov"`.
* GCC: Finds `gcov` to set `GCOV_TOOL="gcov"`.
The `GCOV_TOOL` is then passed to `gcovr` on line 416, so the correct tool is used for processing coverage data.
This change therefore removes the `gcov` suffix from lines 473 and 475 in the `CodeCoverage.cmake` file.
Running unit tests in parallel and multiple threads can write into one file can corrupt output files, and then gcovr won't be able to parse the corrupted file. This change adds -fprofile-update=atomic as instructed by https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68080.
