rr hangs on RHEL7

When running rr record on RHEL7, rr seems to just hang.

Commit 29a59e412ad3e6600fcb37e9a5017f6eafce3b3d introduced this problem. It seems to be getting stuck in the waitid call here. Below is the backtrace from gdb right before it hangs.

(gdb) backtrace
#0  rr::Task::wait_exit (this=0xcd38a0) at /root/rr/src/Task.cc:161
#1  0x00000000008a3d0f in rr::Task::proceed_to_exit (this=0xcd38a0, wait=true) at /root/rr/src/Task.cc:184
#2  0x00000000007b6c9d in rr::handle_ptrace_exit_event (t=0xcd38a0) at /root/rr/src/RecordSession.cc:228
#3  0x00000000007bfbb8 in rr::RecordSession::record_step (this=0xcd1520) at /root/rr/src/RecordSession.cc:2321
#4  0x00000000007b3283 in rr::record (args=std::vector of length 1, capacity 2 = {...}, flags=...)
    at /root/rr/src/RecordCommand.cc:649
#5  0x00000000007b3d5b in rr::RecordCommand::run (this=0xccbeb0 <rr::RecordCommand::singleton>, 
    args=std::vector of length 1, capacity 2 = {...}) at /root/rr/src/RecordCommand.cc:792
#6  0x00000000008fa38b in main (argc=3, argv=0x7fffffffe108) at /root/rr/src/main.cc:268

Reproduce rr record on RHEL7, then start at src/Task.cc:161 and inspect the waitid call in the context of commit 29a59e412ad3e6600fcb37e9a5017f6eafce3b3d. Trace the exit path through src/RecordSession.cc and confirm that recording completes without hanging on RHEL7.