This patch add prologue scan unwinder support for signed (mangled) return
address on AArch64.

aarch64_analyze_prologue is taught about the new "paciasp" and "autiasp"
instructions which are used to sign and authenticate return address.  Prologue
scan unwinder scan their existence to record the signing status of return
address in "ra_state" register which will be used in register unwind method to
restore original value of return address.
                                                                          
I have checked "autiasp" in prologue scanner although it is supposed to be in
epilogue only if the debuggee is compiled from GCC.

gdb/
2017-08-09  Jiong Wang<jiong.wang@arm.com>

	* aarch64-tdep.c (aarch64_analyze_prologue): Recognize "paciasp"
	and "autiasp".  Record signing status of return address in
	"ra_state".
	(aarch64_prologue_prev_register): Mask off signature from return
	address if it's signed.
	(aarch64_analyze_prologue_test): Add new unit test for return
	address signing instruction.