Weak external symbol support

The first commit adds a #[weak] attribute, the second commit makes src/libstd/rt/thread.rs link weakly against __pthread_get_minstack().  Avoids the dlopen/dlsym/dlclose cycle for new threads that was introduced in commit 431edac.

I imagine it's possible that you first need to build rustc using the first commit before you can build the second commit.  I can split this into two pull requests if that is the case.