Work around link problem by explicitly disabling HANDLE_FORK support.

With this I stop hitting obscure link problems, and instead am hitting
"obvious" out-of-memory link problems.

This is the error I am working around:
```
% x86_64-unknown-linux-gnu/stage0/bin/rustc --cfg stage0  -O --cfg rtopt --cfg debug -Z prefer-dynamic --target=x86_64-unknown-linux-gnu  -o x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/bin/rustc /home/pnkfelix/Dev/Mozilla/rust-bdw/src/driver/driver.rs --cfg rustc
...
note: /usr/bin/ld: x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/bin/rustc: hidden symbol `pthread_atfork' in /usr/lib/x86_64-linux-gnu/libpthread_nonshared.a(pthread_atfork.oS) is referenced by DSO
/usr/bin/ld: final link failed: Bad value
collect2: error: ld returned 1 exit status
```

I am basing my workaround on the email dialogue I found here:

  http://www.hpl.hp.com/hosted/linux/mail-archives/gc/2012-March/005069.html