Summary:
This diff adds a `fixup-dyn-deps` subcommand that is intended to
aid in packaging on multiple platforms.

Its purpose is to copy a set of executable object files from the getdeps
installation directories and place them into an installation staging
area that will then be used to create some kind of package (rpm, tarball etc.).

The dynamic dependencies of the executables are determined and also copied
into the destination area, and the important part: the execute is rewritten
such that it will load the deps out of an alternate installation prefix.

The implementation of this command draws on similar scripts in use for
the watchman and eden packaging on windows and macos.  This diff adds
linux support using the `patchelf` utility.

Reviewed By: pkaush

Differential Revision: D16101902

fbshipit-source-id: 5885125971947139407841e08c0cf9f35fdf5895