[AArch64] Refactor aarch64_make_prologue_cache

[AArch64] Refactor aarch64_make_prologue_cache

We would previously have to make sure the frame cache was not already
created before calling aarch64_make_prologue_cache.  This patch makes
this function check it so that the caller does not need to do so.

gdb/ChangeLog:

	* aarch64-tdep.c (aarch64_make_prologue_cache): Update comment.
	New argument this_cache.  Return early if this_cache is already
	set.  Set this_cache.
	(aarch64_prologue_this_id): Update call to
	aarch64_make_prologue_cache.
	(aarch64_prologue_prev_register): Likewise.
	(aarch64_normal_frame_base): Likewise.