[AArch64] Recognize STR instruction in prologue

[AArch64] Recognize STR instruction in prologue

This patch teaches GDB AArch64 backend to recognize STR instructions
in prologue, like 'str x19, [sp, #-48]!' or 'str w0, [sp, #44]'.
The unit test is added too.

gdb:

2016-12-02  Yao Qi  <yao.qi@linaro.org>
	    Pedro Alves  <palves@redhat.com>

	* aarch64-tdep.c (aarch64_analyze_prologue): Recognize STR
	instruction.
	(aarch64_analyze_prologue_test): More tests.