lang/python311: Improve build times

Don't generate debug symbols by default, we discard these anyway and on my
Tigerlake laptop it reduces build time roughly by 25% with LTO enabled
Switch from -flto=thin to (fat)lto, there's no parallization so it's
actually slower and produces slightly larger binaries

PR: 279585
Approved by: portmgr (maintainer timeout, 2+ weeks)