Same with V 1.6.9. Actually I got the error on V 1.6.9 and then update to V 1.6.10
Does that bare minimum sketch also fail to compile?Arduino AVR Boards 1.6.12 included with Arduino IDE 1.6.10 uses LTO, which appears to be the cause of this issue. The problem will likely occur if Arduino AVR Boards 1.6.12 is installed in any Arduino IDE version, not just 1.6.10. So you can try to install Arduino AVR Boards 1.6.11 using Boards Manager and be sure to not update from Arduino AVR Boards 1.6.11, even if the IDE shows an updatable boards notification. You can turn the notifications off by File > Preferences > Check for updates at startup(uncheck).