Issue Description:

Currently if you build libdispatch.so on Linux, the resulting binary does not contain the `RUNPATH` or `NEEDED` commands necessary for loading libswiftCore.so. This
results in scripts loading libdispatch.so crashing at runtime for undefined symbols.

This is not a problem if you're using Swift, and link libswiftCore regardless, but for binaries that are dlopening libdispatch, or just don't care about the implementation details of it, this is required for them to work correctly.

To reproduce this using Docker, run these commands:

$ docker pull swift:3.1
$ docker run -it swift:3.1 /bin/bash
# python -c 'import ctypes; ctypes.CDLL("/usr/lib/swift/linux/libdispatch.so")'

By running this you will see this crash:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python2.7/ctypes/__init__.py", line 362, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: /usr/lib/swift/linux/libdispatch.so: undefined symbol: _TMSV

You can also see that this doesn't happen if you pass libswiftCore with `LD_PRELOAD`:

# LD_PRELOAD=/usr/lib/swift/linux/libswiftCore.so python -c 'import ctypes; ctypes.CDLL("/usr/lib/swift/linux/libdispatch.so")'