ARC: show_regs: lockdep: avoid page allocator...

and use smaller/on-stack buffer instead

The motivation for this change was lockdep splat like below.

| potentially unexpected fatal signal 11.
| BUG: sleeping function called from invalid context at ../mm/page_alloc.c:4317
| in_atomic(): 1, irqs_disabled(): 0, pid: 57, name: segv
| no locks held by segv/57.
| Preemption disabled at:
| [<8182f17e>] get_signal+0x4a6/0x7c4
| CPU: 0 PID: 57 Comm: segv Not tainted 4.17.0+ #23
|
| Stack Trace:
|  arc_unwind_core.constprop.1+0xd0/0xf4
|  __might_sleep+0x1f6/0x234
|  __get_free_pages+0x174/0xca0
|  show_regs+0x22/0x330
|  get_signal+0x4ac/0x7c4     # print_fatal_signals() -> preempt_disable()
|  do_signal+0x30/0x224
|  resume_user_mode_begin+0x90/0xd8

So signal handling core calls show_regs() with preemption disabled but
an ensuing GFP_KERNEL page allocator call is flagged by lockdep.

We could have switched to GFP_NOWAIT, but turns out that is not enough
anways and eliding page allocator call leads to less code and
instruction traces to sift thru when debugging pesky crashes.

FWIW, this patch doesn't cure the lockdep splat (which next patch does).

Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>