The kernel implementation of the fast system call path,
the one invoked by the SYSCALL instruction, is broken for
restarting system calls. A C program demonstrating this is below.

Change the system calls to use INT $0x80 instead, because
that (perhaps slightly slower) system call path actually works.

I filed http://www.freebsd.org/cgi/query-pr.cgi?pr=182161.

The C program demonstrating that it is FreeBSD's fault is below.
It reports the same "Bad address" failures from wait.

#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*);

int
main(void)
{
        int i;
        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(SIGCHLD, &sa, 0);

        for(i=0; i<2; i++)
                pthread_create(0, 0, looper, 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)
{
}

int
mywait4(int pid, int *stat, int options, struct rusage *rusage)
{
        int result;

        asm("movq %%rcx, %%r10; syscall"
                : "=a" (result)
                : "a" (7),
                  "D" (pid),
                  "S" (stat),
                  "d" (options),
                  "c" (rusage));
}

static void*
looper(void *v)
{
        int pid, stat, out;
        struct rusage rusage;

        for(;;) {
                if((pid = fork()) == 0)
                        _exit(0);
                out = mywait4(pid, &stat, 0, &rusage);
                if(out != pid) {
                        printf("wait4 returned %d\n", out);
                }
        }
}

Fixes #6372.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/13582047