aarch64: Avoid redundancy in aarch64-cores.def

aarch64: Avoid redundancy in aarch64-cores.def

The flags fields of the aarch64-cores.def always start with
AARCH64_FL_FOR_<ARCH>.  After previous changes, <ARCH> is always
identical to the previous field, so we can drop the explicit
AARCH64_FL_FOR_<ARCH> and derive it programmatically.

This isn't a big saving in itself, but it helps with later patches.

gcc/
	* config/aarch64/aarch64-cores.def: Remove AARCH64_FL_FOR_<ARCH>
	from the flags field.
	* common/config/aarch64/aarch64-common.cc (all_cores): Add it
	here instead.
	* config/aarch64/aarch64.cc (all_cores): Likewise.
	* config/aarch64/driver-aarch64.cc (all_cores): Likewise.