[AArch64] Refactor aarch64_make_stub_cache

[AArch64] Refactor aarch64_make_stub_cache

We would previously have to make sure the frame cache was not already
created before calling aarch64_make_stub_cache.  This patch makes this
function check it so the caller does not need to do so.

gdb/ChangeLog:

	* aarch64-tdep.c (aarch64_make_stub_cache): Update comment.  New
	argument this_cache.  Remove unused local variables reg and
	unwound_fp.  Return early if this_cache is already set.  Set
	this_cache.
	(aarch64_stub_this_id): Update call to aarch64_make_stub_cache.