Add __ffssi2 implementation to compiler-rt builtins

Summary:
During MIPS implementation work for FreeBSD, John Baldwin (jhb@FreeBSD.org)
found that gcc 6.x emits calls to __ffssi2() when compiling libc and some
userland programs in the base system.

Add it to compiler-rt's builtins, based off of the existing __ffsdi2()
implementation.  Also update the CMake files and add a test case.

Reviewers: howard.hinnant, weimingz, rengolin, compnerd

Reviewed By: weimingz

Subscribers: dberris, mgorny, llvm-commits, emaste

Differential Revision: https://reviews.llvm.org/D31721

llvm-svn: 299675