Create NativeRegisterContext for android-arm64
This patch contains all of the major changes required to get lldb-server running on android-arm64 with some very basic functionalists.
Most of the code for the NativeRegisterContextLinux_arm64 class is copied over form the RegisterContextPOSIX_arm64 class (same approach as the one used for NativeRegisterContext_x86_64). The common code between NativeRegisterContextLinux_arm64 and NativeRegisterContextLinux_x86_64 will be factored out to a common base class with a later commit.