x86/signal: delay calling signals on 32bit

When running some ptrace single step tests on x86-32 machine, the below problem
is triggered:

BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:917
in_atomic(): 1, irqs_disabled(): 0, pid: 1041, name: dummy2
Preemption disabled at:[<c100326f>] do_debug+0x1f/0x1a0

CPU: 10 PID: 1041 Comm: dummy2 Tainted: G        W       4.1.13-rt13 #1
Call Trace:
 [<c1aa8306>] dump_stack+0x46/0x5c
 [<c1080517>] ___might_sleep+0x137/0x220
 [<c1ab0eff>] rt_spin_lock+0x1f/0x80
 [<c1064b5a>] do_force_sig_info+0x2a/0xc0
 [<c106567d>] force_sig_info+0xd/0x10
 [<c1010cff>] send_sigtrap+0x6f/0x80
 [<c10033b1>] do_debug+0x161/0x1a0
 [<c1ab2921>] debug_stack_correct+0x2e/0x35

This happens since 95927475 ("x86, traps: Track entry into and exit
from IST context") which was merged in v4.1-rc1.

Signed-off-by: Yang Shi <yang.shi@linaro.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>