Only emit enabled VNDK libraries

Disabled VNDK libraries (due to arch mismatch) are no longer emitted to
make variables VNDK_CORE_LIBRARIES and LLNDK_LIBRARIES

Bug: 7456955
Test: choosecombo to aosp_walleye and m -j
Find libclang_rt.ubsan_standalone-<arch>-android in VNDK_CORE_LIBRARIES.
Only aarch64 and arm are found.

Change-Id: Iaa134d07513e39390fe34a31fdfe2e327b190996