kvm-unit-tests: Fix x86_64 to use host compiler

kvm-unit-tests requires a multilib compiler for x86_64 as it compiles
32bit boot code.

This patch uses the BR2_HOSTARCH_NEEDS_IA32_COMPILER option to enforce
this and avoids using TARGET_CROSS for x86_64 targets and uses the host
32bit (and 64bit) capable compiler.

Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
[Thomas: reword comment in .mk file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/kvm-unit-tests/Config.in

@@ -1,5 +1,6 @@
 config BR2_PACKAGE_KVM_UNIT_TESTS
 	bool "kvm-unit-tests"
+	select BR2_HOSTARCH_NEEDS_IA32_COMPILER if BR2_x86_64=y
 	depends on BR2_arm || BR2_i386 || BR2_powerpc64 || \
 		BR2_powerpc64le || BR2_x86_64
 	help

package/kvm-unit-tests/kvm-unit-tests.mk

@@ -28,9 +28,16 @@ endif
 
 KVM_UNIT_TESTS_CONF_OPTS =\
 	--arch="$(KERNEL_ARCH)" \
-	--cross-prefix="$(TARGET_CROSS)" \
 	--endian="$(KVM_UNIT_TESTS_ENDIAN)"
 
+# For all architectures but x86-64, we use the target
+# compiler. However, for x86-64, we use the host compiler, as
+# kvm-unit-tests builds 32 bit code, which Buildroot toolchains for
+# x86-64 cannot do.
+ifneq ($(BR2_x86_64),y)
+KVM_UNIT_TESTS_CONF_OPTS += --cross-prefix="$(TARGET_CROSS)"
+endif
+
 define KVM_UNIT_TESTS_CONFIGURE_CMDS
 	cd $(@D) && ./configure $(KVM_UNIT_TESTS_CONF_OPTS)
 endef