Remove the remaining Neon adddi3, subdi3 and negdi2 patterns.  As a result
adddi3, subdi3 and negdi2 can now always be expanded early irrespectively of
whether Neon is available.  Also expand the extenddi patterns at the same
time.  Several Neon arch attributes are no longer used and removed.

Code generation is improved in all cases, saving another 400-500 instructions
from the PR77308 testcase (total improvement is over 1700 instructions with
-mcpu=cortex-a57 -O2).

Bootstrap & regress OK on arm-none-linux-gnueabihf --with-cpu=cortex-a57

    gcc/
	* config/arm/arm.md (neon_for_64bits): Remove.
	(avoid_neon_for_64bits): Remove.
	(arm_adddi3): Always split early.
	(arm_subdi3): Always split early.
	(negdi2): Remove Neon expansion.
	(split zero_extend): Split before reload.
	(split sign_extend): Split before reload.

From-SVN: r274825