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>

Makefile

@@ -102,7 +102,7 @@ export HOSTCC
 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

lib/romlib/Makefile

@@ -5,7 +5,7 @@
 #
 
 AS          = $(CROSS_COMPILE)gcc
-AR          = $(CROSS_COMPILE)ar
+AR          = $(CROSS_COMPILE)gcc-ar
 LD          = $(CROSS_COMPILE)ld
 OC          = $(CROSS_COMPILE)objcopy
 CPP         = $(CROSS_COMPILE)gcc -E

plat/rockchip/rk3399/drivers/m0/Makefile

@@ -41,7 +41,7 @@ LDFLAGS			:= -Wl,--gc-sections -Wl,--build-id=none
 # 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