Turn on Global ThinLTO by default

Apply ThinLTO without optimization (--lto-O0) globally. This saves us
2.24% in binary size for system/lib64, as well as more than 4.68% build
time improvement.

Test: presubmit
Bug: 169004486
Change-Id: I1ed90c2058f9ff1fa8a2d3fd7ac1191c5d98dac1