drd/tests: Extract start_thread which can come from libpthread or libc

drd/tests: Extract start_thread which can come from libpthread or libc

The drd/tests/tc21_pthonce and drd/tests/annotate_barrier tests
would fail if start_thread came from libc (as it does in glibc 2.34)
instead of from libpthread. Extract start_thread in filter_stderr.in
and update the backtraces in annotate_barrier.stderr.exp and in
tc21_pthonce.stderr.exp

Tested against glibc 2.34, 2.33 and 2.17 on x86_64.