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/Makefile b/Makefile
index 2dc2956..3267660 100644
--- a/Makefile
+++ b/Makefile
@@ -102,7 +102,7 @@
 CC			:=	${CROSS_COMPILE}gcc
 CPP			:=	${CROSS_COMPILE}gcc -E
 AS			:=	${CROSS_COMPILE}gcc
-AR			:=	${CROSS_COMPILE}ar
+AR			:=	${CROSS_COMPILE}gcc-ar
 LINKER			:=	${CROSS_COMPILE}ld
 OC			:=	${CROSS_COMPILE}objcopy
 OD			:=	${CROSS_COMPILE}objdump