[profiler] Require an ack of the previous sampling signal before sending another.

[profiler] Require an ack of the previous sampling signal before sending another.

This avoids situations where we could end up filling the signal queue
completely, leading to the undocumented EAGAIN return value from pthread_kill,
which breaks ~everything in the STW/GC code because it thinks an error return
value from pthread_kill means the thread has exited.

The actual reason why this happened is that when a thread is suspended, we mask
all signals except the restart signal for that thread. This means that a
program with a large number of threads or where GC collections could take
longer than usual would end up with tens of thousands of sampling signals
queued up, eventually reaching the limit of the user-level signal queue (see
`ulimit -i` and `RLIMIT_SIGPENDING`).

We only do this on systems where we use real time signals. Non-RT signals do
not have this problem as the kernel will throttle them.