* gcc.c (print_multi_os_directory): New variable.
(option_map): Support --print-multi-os-directory.
(struct prefix_list): Add os_multilib field.
(multilib_os_dir): New variable.
(static_specs): Add multilib_options.
(find_a_file): Add multilib argument. Search in GCC or OS multilib
subdirs if non-zero.
(read_specs, execute): Update callers.
(find_file): Likewise. Don't prefix name with multilib_dir, instead
pass 1 as multilib option.
(display_help): Include --print-multi-os-directory.
(add_prefix): Add os_multilib argument. Initialize pl->os_multilib.
(process_command): Update callers. Handle --print-multi-os-directory.
(do_spec_1) ['D']: Use multilib_os_directory if pl->os_multilib is
set.
(main): Update find_a_file and add_prefix callers.
Handle print_multi_os_directory.
(struct mdswitchstr): New.
(mdswitches, n_mdswitches): New variables.
(used_arg): Add MULTILIB_DEFAULT switches too if they are not
present on the command line nor their mutually incompatible
switches.
(default_arg): Optimize.
(set_multilib_dir): Compute multilib_os_dir. Initialize mdswitches
array.
(print_multilib_info): Only print GCC multilib dir name, not OS
multilib dirname.
* genmultilib: Add osdirnames parameter. Output multilib_options
variable. If osdirnames is specified, output dirnames as
dirname:osdirname.
* mklibgcc.in: Use MULTILIB_OSDIRNAMES, --print-multi-directory
and --print-multi-os-directory instead of SHLIB_SLIBDIR_SUFFIXES
to compute libgcc_s soname and install path.
* Makefile.in (libgcc.mk): Pass MULTILIB_OSDIRNAMES instead of
SHLIB_SLIBDIR_SUFFIXES to mklibgcc.
(s_mlib): Pass MULTILIB_OSDIRNAMES or nothing as last genmultilib
argument.