ARC: build: use $(READELF) instead of hard-coded readelf

The top Makefile defines READELF as the readelf in the cross-toolchains.
Use it rather than the host readelf.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>