Summary:
For compatibility with ARM machines from pre v6, the RTS provides
implementations of certain atomic operations. Previously, these
were only included in the threaded RTS.

But ghc (the library) contains the code in compiler/cbits/genSym.c, which
uses these operations if there is more than one capability. But there is only
one libHSghc, so the linker wants to resolve these symbols in every case.

By providing these operations in all RTSs, the linker is happy. The only
downside is a small amount of dead code in the non-threaded RTS on old ARM
machines.

Test Plan: It helped here.

Reviewers: bgamari, austin

Reviewed By: bgamari, austin

Subscribers: carter, thomie

Differential Revision: https://phabricator.haskell.org/D564

GHC Trac Issues: #8951