Revert^2 "Enable full LTO optimization by default"

55019c43

We set the default optimization mode to --lto-O0 for LTO enabled
projects, in order to save build time. This is missing some performance
optimizations, esp. related to vectorization. Now that we suggest eng
build for developers, we can enable full optimization by default.

When we introduced --lto-O0, we achieved a 4.2% saving in
system-processes-memory-direct. Enabling full LTO optimization will
trade some of the memory / binary size savings for better code
performance. For system-processes-memory-direct, it is 2.0% increase
compared to --lto-O0, or a net 2.2% saving compared to baseline.

Change-Id: I747939ac4d6e4a66e3ef776f4c36eebc7bf34c86