kernel/lib: Adjustments to use musl's ffs implementation.

This actually has another advantage: if we can use musl sources
in the kernel POSIX support, then we can drop some of the BSD
sources that are used only in the kernel and have a different
implementation in userland (e.g. strtod) and have just one
version for both.

libroot: Replace custom ffs implementation with musl's.

It already has a per-arch implementation or a fallback in
musl's own arch_atomic.h, which we already imported so we
might as well leverage it.