Make make_absolute thread-local in chdir.c

[CPunisher]

make_absolute previously used a process-wide static buffer to assemble absolute paths for relative path resolution. In _REENTRANT builds, concurrent calls could overwrite or reallocate that shared buffer while another thread was still using it, leading to races and potentially invalid pointers.

Related #474

[alexcrichton]

Thanks for the PR! I think though this might be something that's more appropriate to use synchronization for rather than thread-local storage. These are malloc'd buffers which means that if it were thread-local then a thread exiting would leak the buffer information. As a static that doesn't matter too much as the leaked memory is bounded, but adding threads into the mix could make it much worse. Could this use a mutex intead?

[CPunisher]

I've updated the code to use synchronization instead of thread-local storage.

I ended up protecting the whole __wasilibc_find_relpath_alloc with a mutex, because the lifetime of make_absolute_buf extends until strcpy(*relative_buf, rel).