[AArch64] Match instruction "STP with base register" in prologue

[AArch64] Match instruction "STP with base register" in prologue

Nowadays, we only match pre-indexed STP in prologue.  Due to the change
in gcc, https://gcc.gnu.org/ml/gcc-patches/2016-07/msg01933.html, it
may generate "STP with base register" in prologue, which GDB doesn't
handle.  That is to say, previously GCC generates prologue like this,

 sub sp, sp, #490
 stp x29, x30, [sp, #-96]!
 mov x29, sp

with the gcc patch above, GCC generates prologue like like this,

 sub sp, sp, #4f0
 stp x29, x30, [sp]
 mov x29, sp

This patch is to teach GDB to recognize this instruction in prologue
analysis.

gdb:

2016-08-19  Yao Qi  <yao.qi@linaro.org>

	* aarch64-tdep.c (aarch64_analyze_prologue): Handle register
	based STP instruction.