Don't use lldb for macos because stdlib tests fail silently with it

[dipinhora]

it seems the signal passing of SIGINT from lldb doesn't behave the same on macos as it does on linux..

[dipinhora]

the issue can be seen in the macos CI builds in PRs for the past few months..

stdlib tests fail early and silently (because lldb doesn't pass through the non-zero program exit code) with the following:

Process 47813 resuming
Process 47813 stopped and restarted: thread 1 received signal: SIGINT
Process 47813 exited with status = 2 (0x00000002) Terminated due to signal 2