Add x86_64 variants to BR2_GCC_TARGET_ARCH

With the Sourcery CodeBench IA32/AMD64 toolchain, the proper -march=
switch must be passed. So, on x86_64, we make sure that
BR2_GCC_TARGET_ARCH gets defined to the correct value, just as we do
on x86.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

target/Config.in.arch

@@ -678,6 +678,11 @@ config BR2_GCC_TARGET_ARCH
 	default athlon-4	if BR2_x86_athlon_4
 	default winchip-c6	if BR2_x86_winchip_c6
 	default winchip2	if BR2_x86_winchip2
+	default nocona		if BR2_x86_64_nocona
+	default core2		if BR2_x86_64_core2
+	default k8		if BR2_x86_64_opteron
+	default k8-sse3		if BR2_x86_64_opteron_sse3
+	default barcelona	if BR2_x86_64_barcelona
 	default c3		if BR2_x86_c3
 	default c3-2		if BR2_x86_c32
 	default geode		if BR2_x86_geode