[AArch64] Use selected cpu's tuning when no tuning parameter is specified

	Backport from mainline
	2014-12-11  Renlin Li  <renlin.li@arm.com>

	* config/aarch64/aarch64.c (aarch64_parse_cpu): Don't define
	selected_tune.
	(aarch64_override_options): Use selected_cpu's tuning.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/branches/gcc-4_9-branch@218634 138bc75d-0d04-0410-961f-82ee72b054a4