getdeps: fixup-dyn-deps: use patchelf from our manifest on linux
getdeps: fixup-dyn-deps: use patchelf from our manifest on linux

Summary:
On linux we use `patchelf` to manipulate dynamic deps but it
isn't guaranteed to be installed everywhere.  We have a manifest file
that describes how to build it, but so far nothing has told getdeps
that it should build it.

This diff updates the ELF dep munging code to literally run
`getdeps.py build patchelf` and then use that patchelf binary to
manipulate the object files.

Refs: https://github.com/facebook/watchman/pull/750

Reviewed By: pkaush

Differential Revision: D17705351

Pulled By: wez

fbshipit-source-id: 358ef239edb389fbd51fa023ff553963aa80b6c7

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst