Add a weak alias from ffs() to __ffssi2() to pacify modern GCC.
GCC 6.2 emits calls to __ffssi2() when compiling libc and some userland
programs in the base system.
Suggested by: jmallett
There may be better ways of doing this. In particular, GCC 6.2 will
probably act the same way when compiling 3rd party applications, so a
symbol in FBSD_private doesn't really seem correct. It seems odd that
GCC 6.2 is even calling __ffssi2() given that older versions did not.