Rename my_ndk_version_root to my_ndk_sysroot; and _include and _lib

prebuilts/ndk/current/platforms/android-19/arch-x86_64/usr/lib
is renamed to usr/lib64 to be more consistent with rest of
lib paths in x86_64 toolchain, which is multilib

See https://android-review.googlesource.com/#/c/92441/

Change-Id: I4e59245505d0fa87ae3608e81e715ccfcecc5ec8