Custom legalization for global_addr.

The code to compute the address of a global variable depends on the kind
of variable, so custom legalization is required.

- Add a legalizer::globalvar module which exposes an
  expand_global_addr() function. This module is likely to grow as we add
  more types of global variables.
- Add a ArgumentPurpose::VMContext enumerator. This is used to represent
  special 'vmctx' arguments that are used as base pointers for vmctx
  globals.

cranelift/docs/langref.rst

@@ -373,7 +373,7 @@ calling convention:
     retlist   : arglist
     arg       : type [argext] [argspecial]
     argext    : "uext" | "sext"
-    argspecial: "sret" | "link" | "fp" | "csr"
+    argspecial: "sret" | "link" | "fp" | "csr" | "vmctx"
     callconv  : `string`
 
 Arguments and return values have flags whose meaning is mostly target