Since the kernel is built with -ffreestanding, and thus -fno-builtin,
the compiler ignores the function names and doesn't perform optimizations
(e.g. inlining for small values) that it otherwise could. So, here,
we add a string.h that supplements the default, and uses #define to
reinstate the builtins, and thus the optimizations.
Linux and FreeBSD at least apparently do the same. A quick compile
benchmark in a VM doesn't show much difference, maybe a slight
decrease in sys time.
Augustin Cavalier
69499b0e00