Proposal: provide cas and barriers symbols even without -threaded

[Ryan Newton]

The "atomic-primops" library depends on symbols such as store_load_barrier
and "cas", which are defined in SMP.h.  Thus the result is that if the
program is linked WITHOUT "-threaded", the user gets a linker error about
undefined symbols.

The specific place it's used is in the 'foreign "C"' bits of this .cmm code:


https://github.com/rrnewton/haskell-lockfree-queue/blob/87e63b21b2a6c375e93c30b98c28c1d04f88781c/AtomicPrimops/cbits/primops.cmm

I'm trying to explore hacks that will enable me to pull in those functions
during compile time, without duplicating a whole bunch of code from the
RTS.  But it's a fragile business.

It seems to me that some of these routines have general utility.  In future
versions of GHC, could we consider linking in those routines irrespective
of "-threaded"?

  -Ryan
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