x86: tracehook_signal_handler

This makes the x86 signal handling code use tracehook_signal_handler() in
place of calling into ptrace guts.  The call is moved after the sa_mask
processing, but there is no other change.  This cleanup doesn't matter to
existing debuggers, but is the sensible thing: have all facets of the
handler setup complete before the debugger inspects the task again.

Signed-off-by: Roland McGrath <roland@redhat.com>