Since the year dot GCC has been passed the `-fno-builtin` option, which
eliminates all compiler knowledge of the C library, even down to basic
stuff like `memcpy` or `memset`, potentially inhibiting quite a lot of
optimisations.
Remove the option to re-enable the optimisations.
There is no record in the source as to why the option is present - maybe
we'll find out by trying to remove it. If necessary, it could be
selectively turned back on for particular functions.
