pyximport: fix a bug crashing other processes

pyxbuild overwrites .so file contents, corrupting any mmapped code image
loaded. If pyximporting the same module from two different processes, on
Linux this leads to crashes with bus error.

This commit fixes the issue by unlinking the file first. On Linux et
al., this does not change the file contents of any open FDs.  On
Windows, unlinking fails, and we just switch to using a different file
name.