Patch description:
With CONFIG_PREEMPT_RT=y and CONFIG_DEBUG_PREEMPT=y
lots of sleeping-in-invalid-context BUG messages are displayed for PPC32 
architecture.
Kernel log:
Freeing unused kernel memory: 108k init
BUG: sleeping function called from invalid context init(1) at 
kernel/rt.c:1446
in_atomic():1 [00000001], irqs_disabled():0
Call trace:
 [c0012010] __might_sleep+0xf0/0xfc
 [c0031fd8] __spin_lock+0x38/0x70
 [c004ab68] lru_cache_add_active+0x20/0xa0
 [c0071c24] install_arg_page+0xf0/0x24c
 [c0071ee8] setup_arg_pages+0x168/0x1d4
 [c0095674] load_elf_binary+0x44c/0x11d8
 [c0072ff4] search_binary_handler+0xfc/0x338
 [c00733c0] do_execve+0x190/0x274
 [c0005b30] sys_execve+0x80/0xe0
 [c0002474] ret_from_syscall+0x0/0x70
 [c00050b8] execve+0x8/0x30
 [c00016a8] init+0x1b0/0x248
 [c0005090] original_kernel_thread+0x48/0x64

This happens because interrupts are not enabled in the realtime preemption
mode by the time kernel delivers signals to processes. This patch enables
interrupts so that realtime mutexes can be acquired in the "right" context.
The original code has been found in CONFIG_PREEMPT_RT support for i386 
architecture.
Thanks.

Signed-off-by: Valentine Barshak <vbarshak@ru.mvista.com>