[lldb] Make TestAArch64AdrpAdd depend on the AArch64 target

LLDB is using LLVM's target-specific disassembler which is only available when
the respective LLVM target has been enabled in the build config.

This patch just skips the test if there is no arm64 target (and its
disassembler) available in the current build config.

Reviewed By: jasonmolenda

Differential Revision: https://reviews.llvm.org/D108145