Makefile: Fix ARCH override

Variables set on the command-line are not overridden by normal
assignments. So when passing ARCH=x86_64 on the command-line, build
fails:

Makefile:227: *** This architecture (x86_64) is not supported in kvmtool.

Use the 'override' directive to force the ARCH reassignment.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Tested-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>
Link: https://lore.kernel.org/r/20220722141731.64039-3-jean-philippe@linaro.org
Signed-off-by: Will Deacon <will@kernel.org>
diff --git a/Makefile b/Makefile
index f0df76f..faae0da 100644
--- a/Makefile
+++ b/Makefile
@@ -115,11 +115,11 @@
 	  -e s/riscv64/riscv/ -e s/riscv32/riscv/)
 
 ifeq ($(ARCH),i386)
-	ARCH         := x86
+	override ARCH = x86
 	DEFINES      += -DCONFIG_X86_32
 endif
 ifeq ($(ARCH),x86_64)
-	ARCH         := x86
+	override ARCH = x86
 	DEFINES      += -DCONFIG_X86_64
 	ARCH_PRE_INIT = x86/init.S
 endif