This change figures out some of the build configuration issues we've
been having with libcxx. The c++ span header is added. Per a Discord
discussion we're now turning off `-g` for the default build mode, so
consider using `make MODE=dbg` or `make MODE=zero` for GDB debugging
which works much better than `MODE=` ever has. Note that the default
build mode has always had very good function call / system call logs
plus you can still use ShowCrashReports() for backtrace. Making this
change ensures cosmocc will better conform to FOSS norms. Lastly the
LoadZipArgs() API has been added to cosmopolitan.a and <cosmo.h>.
