[AArch64] Restore gimple_folding of reduction intrinsics

	* config/aarch64/aarch64.c (TARGET_GIMPLE_FOLD_BUILTIN): Define again.
	* config/aarch64/aarch64-builtins.c (aarch64_gimple_fold_builtin):
	Restore, enable for bigendian, update to use __builtin..._scal...

From-SVN: r216742