Add Global ThinLTO option (2nd try)

Instead of making everything into ThinLTO variant by default (it works
but many Soong tests don't like this, and got bit rot due to lack of
active builder for this configuration), let the default option be
ThinLTO and no LTO be a special variant.

Test: m GLOBAL_THINLTO=true
Test: m
Bug: 195134194
Change-Id: I2fd98061ba55eba1fdfdd056fb2f8c2051fd2553