ARC: Override toplevel default -O2 with -O3

ARC kernels have historically been built with -O3, despite top level
Makefile defaulting to -O2. This was facilitated by implicitly ordering
of arch makefile include AFTER top level assigned -O2.

An upstream fix to top level a1c48bb1 ("Makefile: Fix unrecognized
cross-compiler command line options") changed the ordering, making ARC
-O3 defunct.

Fix that by NOT relying on any ordering whatsoever and use the proper
arch override facility now present in kbuild (ARCH_*FLAGS)

Depends-on: ("kbuild: Allow arch Makefiles to override {cpp,ld,c}flags")
Suggested-by: Michal Marek <mmarek@suse.cz>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: stable@vger.kernel.org # 3.16+
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>