Enable ThinLTO for dex2oat

This helps reducing binary size as well as improve performance of
dex2oat.

On Marlin (lower clocked cores)
  With PGO
  dex2oat binary + libraries size:
   - no-lto:   9.90MB
   - thin-lto: 9.81MB

  dex2oat run time for top 25 apps (with speed filter):
    Average case: -2.83%
    Best case   : -4.65%
    Worst case  :  0.25%, likely noise

  Without PGO
  dex2oat binary + libraries size:
   - no-lto:   9.67MB
   - thin-lto: 9.60MB

  dex2oat run time for top 25 apps (with speed filter):
    Average case : -0.96%
    Best case    : -2.30%
    Worst case   :  0.52%, likely noise

Test: Benchmark on Marlin (low-clock cores)
Bug: 62839002
Change-Id: Iad39eae4c222b753dd2dab949a928151ea13e630