fuzzing: Fix the oss-fuzz coverage build

fuzzing: Fix the oss-fuzz coverage build

It was long thought that the issue here was that no seed corpus was
provided, but actually the issue is that to obtain coverage output
just as we already know for gcc gcov, you must provide fuzzing flags
to both the compile and link phase.

Thankfully clang as a linker does not mind the strange non-linker options
from $COVERAGE_FLAGS.

REF: https://stackoverflow.com/questions/56112019/clang-does-not-generate-profraw-file-when-linking-manually
REF: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=19495#c48

Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Wed Oct 21 23:07:37 UTC 2020 on sn-devel-184