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>

arch/arc/boot/Makefile

@@ -6,7 +6,7 @@
 
 OBJCOPYFLAGS= -O binary -R .note -R .note.gnu.build-id -R .comment -S
 
-LINUX_START_TEXT = $$(readelf -h vmlinux | \
+LINUX_START_TEXT = $$($(READELF) -h vmlinux | \
 			grep "Entry point address" | grep -o 0x.*)
 
 UIMAGE_LOADADDR    = $(CONFIG_LINUX_LINK_BASE)