Revert "Use synchronize chdir for unix_workers and remove synchronize for pwd"

There could be a chance that another thread can be chdir,
while Dir.pwd is called

This reverts commit 9fe73f63487a29e730e70e766999dcb3ec7dea9f.
Removing monkey patch to rubygems build_extensions

As of Rubygems 2.0.7 build_extensions uses mutex
to ensure thread-saftey, so no need to duplicate
efforts in Bundler.