Link 32bit native libraries only if libs exist

There could be an app whose abi is 32bit, but which doesn't have 32bit
native libs because it just belongs to the SharedUserId whose abi is
32bit. Link it only if libs exist to reduce unnecessary calls.

Bug: 204097684
Test: compile & verify basic functions working
Change-Id: I24c374c92600ff50f4cf4e88e0b4106d480562d8