Break false dependencies on target libraries

Break false dependencies on target libraries

Summary:
For the most part this consists of replacing ${LLVM_TARGETS_TO_BUILD} with
some combination of AllTargets* so that they depend on specific components
of a target backend rather than all of it. The overall effect of this is
that, for example, tools like opt no longer falsely depend on the
disassembler, while tools like llvm-ar no longer depend on the code
generator.

There's a couple quirks to point out here:
* AllTargetsCodeGens is a bit more prevalent than expected. Tools like dsymutil
  seem to need it which I was surprised by.
* llvm-xray linked to all the backends but doesn't seem to need any of them.
  It builds and passes the tests so that seems to be correct.
* I left gold out as it's not built when binutils is not available so I'm
  unable to test it

Reviewers: bogner, JDevlieghere

Reviewed By: bogner

Subscribers: mehdi_amini, mgorny, steven_wu, dexonsmith, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62331

llvm-svn: 361567