clang 3.8 flags -Wshift-negative-value warning, which turns fatal due to use of -Werror. Fixes: https://github.com/tianocore/edk2/issues/49 Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Zenith432 <zenith432@users.sourceforge.net> Reviewed-by: Liming Gao <liming.gao@intel.com>