The following should lead to a normal error message, not an ICE:

7910:tbm@deprecation: ~/tmp/src] gcc -imultilib t.c
cc1: internal compiler error: no multiarch mapping for multilib (t.c)

Please submit a full bug report,
with preprocessed source if appropriate.
See <URL:http://gcc.gnu.org/bugs.html> for instructions.
For Debian GNU/Linux specific bug reporting instructions,
see <URL:file:///usr/share/doc/gcc-4.1/README.Bugs>.
zsh: exit 1     gcc -imultilib t.c
7911:tbm@deprecation: ~/tmp/src] cat t.c
int main() {
}
7912:tbm@deprecation: ~/tmp/src]