Add an implementation of ffssi2() derived from ffsdi2().

Newer versions of GCC include an __ffssi2() symbol in libgcc and the
compiler can emit calls to it in generated code. This is true for at
least GCC 6.2 when compiling world for mips and mips64.

Reviewed by: jmallett, dim
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D10086