Backport of "cargo: Default to thin lto for the release profile" into 0.9

Normal "fat" lto is quite resource heavy and requires a lot of ram at compile time. Thinlto is a much lighter alternative that produces comparable results performance but it scales much better. 1 2 3

All browser engines also compile with thinlto exclusively, as it's almost as fast and saves considerable amount of link time in the build.

In a test build on a windows machine, this brings down the from scratch compile time from 7m30s to roughly 5m.

Part-of: !1924 (merged)