No atomics on arm32; this will just yield stubs.

As such the internal linker will fail for them.  The alternative
would be to implement them as stubs in the linker and have them
barf when called.

> Not all operations are supported by all target processors. If a
  particular operation cannot be implemented on the target processor,
  a warning is generated and a call an external function is
  generated. The external function carries the same name as the
  built-in version, with an additional suffix ‘_n’ where n is the size
  of the data type.

(https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html)

libraries/ghc-prim/cbits/atomic.c

+#if !defined(arm_HOST_ARCH)
 #include "Rts.h"
 
 // Fallbacks for atomic primops on byte arrays. The builtins used
@@ -418,3 +419,4 @@ hs_atomicwrite64(StgWord x, StgWord64 val)
   while (!__sync_bool_compare_and_swap((StgWord64 *) x, *(StgWord64 *) x, (StgWord64) val));
 #endif
 }
+#endif