Log Message:
Change the way that ${MULTILIB_SUPPORTED} is used.

Three situations need it be handled:

1) Multilib support is unknowen, i.e. there is nothing in the options.mk
   file to appropriately set ${MULTILIB_SUPPORTED} (currently all platforms
   except Linux/x86_64).  In this situation nothing should be done.

2) Multilib _is_ supported, in this situation the 'gcc-multilib' option
   should be made available and the CONFIGURE_ARGS modified accordingly.

3) Multilib _is not_ supported, in this situation CONFIGURE_ARGS need to
   be modified.