[WebAssembly] Make stack-pointer imports mutable.

This is not currently valid by the wasm spec, however:
 - It replaces doing set_global on an immutable global, which is also
   not valid.
 - It's expected be valid in the near future:
   https://github.com/WebAssembly/threads/blob/master/proposals/threads/Globals.md
 - This only occurs before linking, so a fully linked object will be
   valid.

llvm-svn: 319810