[AArch64] Only access inferior registers when creating a frame cache

[AArch64] Only access inferior registers when creating a frame cache

This patch moves the address of the start of a function (func) and the
address from which it was called (prev_pc) into aarch64_prologue_cache.
The idea is to keep accesses to the inferior's registers into
aarch64_make_prologue_cache and aarch64_make_stub_cache.

gdb/ChangeLog:

	* aarch64-tdep.c (aarch64_prologue_cache) <func, prev_pc>: New
	fields.
	(aarch64_scan_prologue): Set prev_pc.
	(aarch64_make_prologue_cache): Set func.
	(aarch64_make_stub_cache): Set prev_pc.
	(aarch64_prologue_this_id): Remove local variables id, pc and
	func.  Read prev_pc and func from cache.
	(aarch64_stub_this_id): Read prev_pc from cache.