Emit LOCAL_MODULE_TARGET_ARCH for prebuilt_etc

Absense of the flag causes problem when prebuilt_etc is configured with
compile_multilib.

Bug: 130138217
Test: choosecombo to aosp_x86_64 and build
test_com.android.media.swcodec
Check that the test APEX has 32-bit artifacts.

Merged-In: Idd28443d129ff70053295015e69328a8fa3eca47
Change-Id: I6b90668d6effc9f82f7cf3a958d4964e8f06ac8e