[libc++] Fix tests broken on the Buildkite CI (#146733)

The Buildkite CI was unintentionally disabled for a few weeks. This
patch fixes the
CI jobs now that is has been re-enabled.

Author: ldionne

libcxx/test/std/thread/futures/futures.async/thread_create_failure.pass.cpp

@@ -16,9 +16,10 @@
 // There is no way to limit the number of threads on windows
 // UNSUPPORTED: windows
 
-// AIX and macOS seem to limit the number of processes, not threads via RLIMIT_NPROC
+// AIX, macOS and FreeBSD seem to limit the number of processes, not threads via RLIMIT_NPROC
 // XFAIL: target={{.+}}-aix{{.*}}
 // XFAIL: target={{.+}}-apple-{{.*}}
+// XFAIL: freebsd
 
 // This test makes sure that we fail gracefully in care the thread creation fails. This is only reliably possible on
 // systems that allow limiting the number of threads that can be created. See https://llvm.org/PR125428 for more details