Remove GLOBAL_THINLTO build option

GLOBAL_THINLTO is now default, since it improves build speed, produces
more performant code and smaller binary. Remove the option to turn it
off since we no longer maintain that variation, and that simplifies the
logic (esp. for moving CFI builds to ThinLTO).

Also fixed a bug where ThinLTO is not propagated to its static deps on
non-default targets (32-bit or host).

Test: presubmit
Bug: 169004486
Change-Id: I31f41ba27c2b94a384d2ba5027049c307d6f4334