[WebAssembly] Add assembly syntax for mutable globals

This adds and optional ", immutable" to the end of a `.globaltype`
declaration.  I would have prefered to match the `.wat` syntax
where immutable is the default and `mut` is the signifier for
mutable globals.  Sadly changing the default would break backwards
compat with existing assembly in the wild so I think its best
to stick with this approach.

Differential Revision: https://reviews.llvm.org/D87515