init: Build init_tests only for the first arch.

Not all the dependent shared libs are installed on device with both
arches. For example, we don't have 32-bit `libfs_mgr.so` or
`libbootloader_message.so` on marlin, which would cause 32-bit
`init_tests` to fail.

Bug: 112494634
Test: Run (64-bit) init_tests on marlin.
Test: `m -j continuous_native_tests` with aosp_marlin-userdebug. Check
      that only 64-bit init_tests is included in
      `continuous_native_tests.zip`.
Change-Id: I999d58750d48c34df7dbabccf8863a6ccaf0b83c