amd64: set UBOOT_COMPILER for amd64/x86

- this is so we can build qemu-uboot-x86 on arm64

config/sources/amd64.conf

@@ -33,6 +33,7 @@ if [[ "$(uname -m)" == "aarch64" ]]; then
 	# Allow building amd64 on aarch64, but using system toolchain only
 	declare -g KERNEL_COMPILER="x86_64-linux-gnu-"
 	declare -g SKIP_EXTERNAL_TOOLCHAINS=yes
+	declare -g UBOOT_COMPILER='x86_64-linux-gnu-'
 fi
 
 true # make sure to exit with 0 status; this protects against shortcircuits etc above.