[WebAssembly] Define the atomic type sizes

WebAssembly's spec has now been updated to specify some guarantees
about lock free atomic accesses. Update clang to match.

This also updates sig_atomic_t to be 64-bit on wasm64. WebAssembly
does not presently have asynchronous interrupts, but this change is
within the spirit of how they will work if they are added.

Differential Revision: http://reviews.llvm.org/D12862

llvm-svn: 247624