relocatable-prog: Add support for 32-bit build on FreeBSD/powerpc64. Reference: <https://savannah.gnu.org/patch/?10469>. * build-aux/config.libpath: Accept a second argument. Use it on FreeBSD/powerpc64. (func_usage): Document it. * m4/relocatable.m4 (AC_LIB_LIBPATH): Require gl_HOST_CPU_C_ABI_32BIT. Pass HOST_CPU_C_ABI_32BIT to the config.libpath script. * modules/relocatable-prog (Files): Add m4/host-cpu-c-abi.m4.