[ARM] minor opt in thumb_stack_frame_destroyed_p

thumb_stack_frame_destroyed_p scans the instructions from PC to the
end of the function, but if PC is far from the end of pc, we don't
have to scan, because PC should be in epilogue if it is still
far from the end of the function.  The criterion I use here is 16
bytes, which is more than 4 instructions.

Regression tested on aarch64-linux with mutli-arch debug.

gdb:

2016-04-15  Yao Qi  <yao.qi@linaro.org>

	* arm-tdep.c (thumb_stack_frame_destroyed_p): Return zero if
	PC is far from the end of function.