[3.1 blocker] win32.S needs to handle relocations/GOT

From a Mozilla trybot:
```
/builds/slave/try-lx-00000000000000000000000/build/gcc/bin/../lib/gcc/x86_64-unknown-linux-gnu/4.7.3/../../../../x86_64-unknown-linux-gnu/bin/ld: read-only segment has dynamic relocations.
```
win32.S needs to handle relocations/GOT/@PLT like sysv.S does.  Ideally it should do so using portability macros that make it easy to disable this on other platforms.