Fix gdb.base/watch-vfork.exp: Watchpoint triggers after vfork (sw) (timeout) with...

Fix gdb.base/watch-vfork.exp: Watchpoint triggers after vfork (sw) (timeout) with Linux 2.6.32 and older version
https://sourceware.org/ml/gdb-patches/2014-04/msg00047.html

Got gdb.base/watch-vfork.exp: Watchpoint triggers after vfork (sw)
(timeout) with Linux 2.6.32 and older version.

The rootcause is after the test use "set can-use-hw-watchpoints 0" let GDB
doesn't use hardware breakpoint and set a watchpoint on "global", GDB
continue will keep single step inside function "vfork".
The Linux 2.6.32 and older version doesn't have commit
6580807da14c423f0d0a708108e6df6ebc8bc83d (get more info please goto
http://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/commit/?id=6580807da14c423f0d0a708108e6df6ebc8bc83d).
When the function "vfork" do syscall, the single step flag TIF_SINGLESTEP
will copy to child process.
Then GDB detach it, child process and parent process will be hanged.

So I make a patch that do a single step before detach.  Then TIF_SINGLESTEP
of child process in old Linux kernel will be cleared before detach.
Child process in new Linux kernel will not be affected by this single step.

2014-06-08  Hui Zhu  <hui@codesourcery.com>

	* common/linux-ptrace.c (linux_disable_event_reporting): New
	function.
	* common/linux-ptrace.h (linux_disable_event_reporting): New
	declaration.
	* linux-nat.c (linux_child_follow_fork): Do a single step before
	detach.