build: prefer `gcc-ar` over `ar`

The `gcc-ar` wrapper exists to make it easier to support LTO on some
versions of GCC. The two commands are compatible, accepting exactly the
same arguments, so this change moves us to `gcc-ar` to ensure that we
are configuring LTO correctly.

Change-Id: I24a4cfaad29d35b09f847299081f83ca9b41aa8a
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/plat/rockchip/rk3399/drivers/m0/Makefile b/plat/rockchip/rk3399/drivers/m0/Makefile
index 1763246..8783269 100644
--- a/plat/rockchip/rk3399/drivers/m0/Makefile
+++ b/plat/rockchip/rk3399/drivers/m0/Makefile
@@ -41,7 +41,7 @@
 # Cross tool
 CC			:= ${M0_CROSS_COMPILE}gcc
 CPP			:= ${M0_CROSS_COMPILE}gcc -E
-AR			:= ${M0_CROSS_COMPILE}ar
+AR			:= ${M0_CROSS_COMPILE}gcc-ar
 OC			:= ${M0_CROSS_COMPILE}objcopy
 OD			:= ${M0_CROSS_COMPILE}objdump
 NM			:= ${M0_CROSS_COMPILE}nm