Revert "build/core: Use better default compiler flags"

This reverts commit f89bcfb8e5b1b4e8320d0a3e2d89b668b1988596.
Make this commit opt-in: f89bcfb8e5b1b4e8320d0a3e2d89b668b1988596

To enable export USE_MORE_OPT_FLAGS=yes

Set LD flags to something safe by default.