Revert "defconfig: Don't use -Os"

This reverts commit 9fb057f05168de0f43e832ad762f0d4427e16ef8.
Revert "DERP"

This reverts commit 87d57fbe842c42c850d80d070e22f23661131854.
Revert "Makefile: Use -O3 where possible"

This reverts commit a10f8be8c8044ab548aff065c9061b9548a65ff1.