Fix build break on Ubuntu 12.04 with ARM64 changes.

See http://llvm.org/bugs/show_bug.cgi?id=20824 for more details.

Tested:
Ubuntu 12.04 x86_64, gcc-4.9.1-built lldb
Ubuntu 14.04 x86_64, clang-3.5-build lldb



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@217169 91177308-0d34-0410-b5e6-96231b3b80d8