cmake: don't use `-fcoroutines` for clang
cmake: don't use `-fcoroutines` for clang

This gcc flag is not supported. `-fcoroutines-ts` also
cannot be used, so just don't supply anything, similar to
what `configure.py` does.

Signed-off-by: Pavel Solodovnikov <2cf5887fafb117b7ff3e29b9720a7305c3313494@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla