rts: always build 64-bit atomic ops

This patch does a few things:

- Always build 64-bit atomic ops in rts/ghc-prim, even on 32-bit
  platforms
- Remove legacy "64bit" cabal flag of rts package
- Fix hs_xchg64 function prototype for 32-bit platforms
- Fix AtomicFetch test for wasm32

(cherry picked from commit 87095f6a)