Alex Biehl authored Jul 30, 2016 and Erik de Castro Lopo committed Jul 30, 2016

This patch resulted from the discussion in D2431 and should be merged
first.

@erikd and @trommler reported errors like

```
/home/erikd/Git/ghc-upstream/rts/dist/build/libHSrts_thr.a(PrimOps.thr_o
): In function `c14_info':
(.text+0x2b8): undefined reference to `hs_cmpxchg32'
/home/erikd/Git/ghc-upstream/rts/dist/build/libHSrts_thr.a(PrimOps.thr_o
): In function `c5e_info':
(.text+0xac4): undefined reference to `hs_cmpxchg32'
/home/erikd/Git/ghc-upstream/rts/dist/build/libHSrts_thr.a(PrimOps.thr_o
): In function `c8b_info':
(.text+0x1198): undefined reference to `hs_cmpxchg32'
/home/erikd/Git/ghc-upstream/rts/dist/build/libHSrts_thr.a(PrimOps.thr_o
): In function `c8b_info':
(.text+0x122c): undefined reference to `hs_cmpxchg32'
/home/erikd/Git/ghc-upstream/rts/dist/build/libHSrts_thr.a(PrimOps.thr_o
): In function `c8b_info':
(.text+0x12ec): undefined reference to `hs_cmpxchg32'
```

on PowerPC. @simonmar suggests to add the specific exports to
`rts/package.conf.in`. This patch does exactly that, including all
other atomic ops as they probably (maybe someone can verify?) suffer
from the same problem on PPC.

Test Plan: Please make sure to build on PPC.

Reviewers: erikd, austin, bgamari, simonmar, trommler

Reviewed By: erikd, trommler

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2435