PR remote/19496, interrupted syscall in forking-threads-plus-bkpt

PR remote/19496, interrupted syscall in forking-threads-plus-bkpt

This patch addresses "fork:Interrupted system call" (or wait:) failures
in gdb.threads/forking-threads-plus-breakpoint.exp.

The test program spawns ten threads, each of which do ten fork/waitpid
sequences.  The cause of the problem was that when one of the fork
children exited before the corresponding fork parent could initiate its
waitpid for that child, a SIGCHLD and/or SIGSTOP was delivered and
interrupted a fork or waitpid in another thread.

The fix was to wrap the system calls in a loop to retry the call if
it was interrupted, like:

do
  {
    pid = fork ();
  }
while (pid == -1 && errno == EINTR);

Since this is a Linux-only test I figure it is OK to use errno and EINTR.
I tried a number of alternative fixes using SIG_IGN, SA_RESTART,
pthread_sigblock, and bsd_signal, but none of these worked as well.

Tested on Nios II Linux target with x86 Linux host.

gdb/testsuite/ChangeLog:
2016-03-16  Don Breazeal  <donb@codesourcery.com>

	* gdb.threads/forking-threads-plus-breakpoint.c (thread_forks):
	Retry fork and waitpid on interrupted system call errors.
	* gdb.threads/forking-threads-plus-breakpoint.exp: (do_test):
	Use with_timeout_factor to increase timeout to 90.