AArch64: Prologue scan unwinder support for signed return addresses

AArch64: Prologue scan unwinder support for signed return addresses

Pauth address signing is enabled at binary compile time.  When enabled the
return addresses for functions may be mangled.  This patch adds functionality
to restore the original address for use in the prologue scan unwinder.

In the prologue analyzer, check for PACIASP/PACIBSP (enable address mangling)
and AUTIASP/AUTIBSP (disable address mangling).

When unwinding the PC from the prologue, unmask the register if required.

Add a test case to the prologue tests.

gdb/ChangeLog:

	* aarch64-tdep.c (aarch64_analyze_prologue): Check for pauth
	instructions.
	(aarch64_analyze_prologue_test): Add PACIASP test.
	(aarch64_prologue_prev_register): Unmask PC value.