MULTILIB_DEFAULTS need an update for ARM multilib builds

I did see gcc-4.7 fail to build for an sf/hf multilib configuration. The reason was that gcc -print-multi-directory didn't print anything for the non-default, and gcc -print-multi-lib only prints `.'. The reason is that set_multilib_dir in gcc.c only consults MULTILIB_DEFAULTS (only defined in linux-elf.h), but not the default configure options in configargs.h. This proposed patch updates MULTILIB_DEFAULTS depending on the configure options. An alternative approach would be to update set_multilib_dir and print_multilib_info to lookup the configure_default_options in configargs.h as well.

Note that this didn't fail to build in gcc-4.6, but I can't see yet what change did cause the build failure.

Matthias