Move LLNDK ABI dumps from vndk/ to platform/

Since vndk libraries don't need to maintain Cross-Version abi
compatibility. The newly proposed Cross-Version ABi checking mechanism
is applied to only ndk/ and platform/. The reference dumps of LLNDK
libraries should get from platform/ to be benefited from this checking
mechanism.

Test: mkdir -p prebuilts/abi-dumps/platform/UpsideDownCake/64/x86_64/source-based/ && cp prebuilts/abi-dumps/vndk/32/64/x86_64/source-based/libvndksupport.so.lsdump "$_"
      make libvndksupport -j
Bug: 238383845
Change-Id: I663511d60623cd23619166d8d63fd26220a930e2