Revert "Revert "Set global default to use lld.""

This reverts commit 44988be8.

Three known build failures with lld are fixed by
(1) adding --no-fatal-warnings to link
(2) disable LLD for some PGO build targets
(3) disable LLD for some art oatdump tests, pending c/681542

Change-Id: Icebf6661509bf4f6051f91dc74eaa07dbc7c1d42