Enable ThinLTO for libart

This helps reducing binary size as well as improve performance of
libart.

On Marlin
                      No LTO    ThinLTO   Improvement
  libart.so           5.82MB    5.66MB    2.75%
  libart.so (64)      7.71MB    7.44MB    3.44%
  atest (AppStartup)  270.13    265.13    1.85%
  dex2oat             536.04    533.03    0.56%

Bug: 62839002
Test: Benchmark on Marlin
Change-Id: Ic386ebceecf89239ad4f28db28b9bfd641efc45c