arm64: clean up tools Makefile

arm64: clean up tools Makefile

Remove unused gen-y.

Remove redundant $(shell ...) because 'mkdir' is done in cmd_gen_cpucaps.

Replace $(filter-out $(PHONY), $^) with the $(real-prereqs) shorthand.

The '&&' in cmd_gen_cpucaps should be replaced with ';' because it is
run under 'set -e' environment.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20220227085232.206529-1-masahiroy@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>