Partially fix C++ coverage on Linux
Partially fix C++ coverage on Linux

gcov 8.3.0 no longer prints "Creating <output>" messages, so we have to
fall back to collecting all .gcov files in the current directory. Also,
provide a mechanism to pass additional options to gcov, e.g.:

--test_env=COVERAGE_GCOV_OPTIONS="-p -x"

We cannot always pass these flags, as not all versions of gcov support
them.

This also solves one issue on MacOS, where gcov output emits lower-case
"creating <output>" messages.

Progress on #9406.

PiperOrigin-RevId: 291172263

ulfjack/bazel,dslomov/bazel-windows,dslomov/bazel-windows,cushon/bazel,dslomov/bazel-windows,werkt/bazel,dslomov/bazel,meteorcloudy/bazel,dslomov/bazel-windows,safarmer/bazel,meteorcloudy/bazel,dslomov/bazel-windows,perezd/bazel,safarmer/bazel,werkt/bazel,ButterflyNetwork/bazel,werkt/bazel,perezd/bazel,cushon/bazel,davidzchen/bazel,ulfjack/bazel,akira-baruah/bazel,meteorcloudy/bazel,twitter-forks/bazel,bazelbuild/bazel,akira-baruah/bazel,ulfjack/bazel,bazelbuild/bazel,bazelbuild/bazel,davidzchen/bazel,twitter-forks/bazel,werkt/bazel,davidzchen/bazel,perezd/bazel,akira-baruah/bazel,dslomov/bazel,meteorcloudy/bazel,ButterflyNetwork/bazel,meteorcloudy/bazel,davidzchen/bazel,davidzchen/bazel,twitter-forks/bazel,dslomov/bazel,perezd/bazel,cushon/bazel,ButterflyNetwork/bazel,bazelbuild/bazel,cushon/bazel,ulfjack/bazel,twitter-forks/bazel,werkt/bazel,cushon/bazel,ButterflyNetwork/bazel,katre/bazel,safarmer/bazel,katre/bazel,davidzchen/bazel,akira-baruah/bazel,safarmer/bazel,safarmer/bazel,ulfjack/bazel,werkt/bazel,katre/bazel,bazelbuild/bazel,safarmer/bazel,akira-baruah/bazel,bazelbuild/bazel,katre/bazel,ulfjack/bazel,cushon/bazel,twitter-forks/bazel,perezd/bazel,katre/bazel,dslomov/bazel-windows,dslomov/bazel,ButterflyNetwork/bazel,akira-baruah/bazel,katre/bazel,twitter-forks/bazel,ulfjack/bazel,perezd/bazel,perezd/bazel,dslomov/bazel,dslomov/bazel,dslomov/bazel,davidzchen/bazel,twitter-forks/bazel,ButterflyNetwork/bazel,meteorcloudy/bazel,meteorcloudy/bazel