Fix cross-compiling the zig compiler

zig0 is only used for building objects, thus it has no options like
`build-exe/obj`. But when cross-compiling, we have a working zig
compiler on the host, thus we need to pass `build-obj` to the zig compiler.

CMakeLists.txt

@@ -493,7 +493,7 @@ if("${ZIG_TARGET_TRIPLE}" STREQUAL "native")
   endif()
 else()
   add_custom_target(zig_build_zig1 ALL
-      COMMAND "${ZIG_EXECUTABLE}" ${BUILD_ZIG1_ARGS}
+      COMMAND "${ZIG_EXECUTABLE}" "build-obj" ${BUILD_ZIG1_ARGS}
       BYPRODUCTS "${ZIG1_OBJECT}"
       COMMENT STATUS "Building self-hosted component ${ZIG1_OBJECT}"
       WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"