Turn on Global ThinLTO by default

Build C/C++ targets with ThinLTO with "--lto-O0" by default. This takes
advantage of better dead code elimination and CFG simplification due to
ThinLTO's global view of the program, but do not enable the costly LTO
optimizations. This also makes builds faster because we can avoid doing
codegen for the dead code.

Code size for panther-userdebug:
                   /system/bin  /system/lib  /system/lib64
Original           57648        86264        181700
Global ThinLTO     55764        84916        175728
                   -3.27%       -1.56%       -3.29%

Build time for aosp_arm64-userdebug:
                   clang time   linker time
Original           56993.87s    1712.36s
Global ThinLTO     52839.18s    3114.93s

saving 4.68% build time in total.

Bug: 169004486
Test: presubmit
Change-Id: Idb0f4675ca4750b8b12b24f4679579419d3448c8