lang/gcc*: disable MULTILIB on current

[brooks]

Summary

If libsys is present, don't enable the MULTILIB option. A bug in
GNU ld prevents gcc -m32 from working without explict overlinking.
While we debug that issue, keep gcc* ports building.

[brooks]

Without this, gcc ports will break again when I commit D43910.

[brooks]

I think I understand the issue and may have found a solution so let's not commit this just yet.

[salvadore]

In case you need to commit it, I approve it. Thanks.

[brooks]

I've worked around this by adding rpath to the libraries.