Fixes #1641.

Actually it side steps the real issue, which is that the
setitimer(2) implementation on OS X is not useful for
profiling of multi-threaded programs.  I filed the below
using the Apple Bug Reporter.

/*
Filed as Apple Bug Report #9177434.

This program creates a new pthread that loops, wasting cpu time.
In the main pthread, it sleeps on a condition that will never come true.
Before doing so it sets up an interval timer using ITIMER_PROF.
The handler prints a message saying which thread it is running on.

POSIX does not specify which thread should receive the signal, but
in order to be useful in a user-mode self-profiler like pprof or gprof
   http://code.google.com/p/google-perftools
   http://www.delorie.com/gnu/docs/binutils/gprof_25.html
it is important that the thread that receives the signal is the one
whose execution caused the timer to expire.

Linux and FreeBSD handle this by sending the signal to the process's
queue but delivering it to the current thread if possible:

   http://lxr.linux.no/linux+v2.6.38/kernel/signal.c#L802
     807        /*
     808         * Now find a thread we can wake up to take the signal off the queue.
     809         *
     810         * If the main thread wants the signal, it gets first crack.
     811         * Probably the least surprising to the average bear.
     812         * /

   http://fxr.watson.org/fxr/source/kern/kern_sig.c?v=FREEBSD8;im=bigexcerpts#L1907
     1914         /*
     1915          * Check if current thread can handle the signal without
     1916          * switching context to another thread.
     1917          * /

On those operating systems, this program prints:

    $ ./a.out
    signal on cpu-chewing looper thread
    signal on cpu-chewing looper thread
    signal on cpu-chewing looper thread
    signal on cpu-chewing looper thread
    signal on cpu-chewing looper thread
    signal on cpu-chewing looper thread
    signal on cpu-chewing looper thread
    signal on cpu-chewing looper thread
    signal on cpu-chewing looper thread
    signal on cpu-chewing looper thread
    $

The OS X kernel does not have any such preference.  Its get_signalthread
does not prefer current_thread(), in contrast to the other two systems,
so the signal gets delivered to the first thread in the list that is able to
handle it, which ends up being the main thread in this experiment.
http://fxr.watson.org/fxr/source/bsd/kern/kern_sig.c?v=xnu-1456.1.26;im=excerpts#L1666

    $ ./a.out
    signal on sleeping main thread
    signal on sleeping main thread
    signal on sleeping main thread
    signal on sleeping main thread
    signal on sleeping main thread
    signal on sleeping main thread
    signal on sleeping main thread
    signal on sleeping main thread
    signal on sleeping main thread
    signal on sleeping main thread
    $

The fix is to make get_signalthread use the same heuristic as
Linux and FreeBSD, namely to use current_thread() if possible
before scanning the process thread list.

*/

#include <sys/time.h>
#include <sys/signal.h>
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

static void handler(int);
static void* looper(void*);

static pthread_t pmain, ploop;

int
main(void)
{
        struct itimerval it;
        struct sigaction sa;
        pthread_cond_t cond;
        pthread_mutex_t mu;

        memset(&sa, 0, sizeof sa);
        sa.sa_handler = handler;
        sa.sa_flags = SA_RESTART;
        memset(&sa.sa_mask, 0xff, sizeof sa.sa_mask);
        sigaction(SIGPROF, &sa, 0);

        pmain = pthread_self();
        pthread_create(&ploop, 0, looper, 0);

        memset(&it, 0, sizeof it);
        it.it_interval.tv_usec = 10000;
        it.it_value = it.it_interval;
        setitimer(ITIMER_PROF, &it, 0);

        pthread_mutex_init(&mu, 0);
        pthread_mutex_lock(&mu);

        pthread_cond_init(&cond, 0);
        for(;;)
                pthread_cond_wait(&cond, &mu);

        return 0;
}

static void
handler(int sig)
{
        static int nsig;
        pthread_t p;

        p = pthread_self();
        if(p == pmain)
                printf("signal on sleeping main thread\n");
        else if(p == ploop)
                printf("signal on cpu-chewing looper thread\n");
        else
                printf("signal on %p\n", (void*)p);
        if(++nsig >= 10)
                exit(0);
}

static void*
looper(void *v)
{
        for(;;);
}

R=r
CC=golang-dev
https://golang.org/cl/4273113