[CMake] Revert -Wl,-O3

This reverts 8cb7876c and follow-ups.

GNU ld/gold/ld.lld -O has nothing to do with any code related linker optimizations.
It has very small benefit (save 144Ki (.hash, .gnu_hash) with GNU ld, save 0.7%
.debug_str with gold/ld.lld) while it makes gold/ld.lld significantly slower
when linking RelWithDebInfo clang (gold: 16.437 vs 19.488; ld.lld: 1.882 vs 4.881).