[Disassembler] Run ARM-specific tests only if the ARM backend is built.
[Disassembler] Run ARM-specific tests only if the ARM backend is built.

<rdar://problem/44239070>

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@341714 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb