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.

lib/cretonne/src/legalizer/mod.rs

@@ -22,8 +22,11 @@ use isa::TargetIsa;
 use bitset::BitSet;
 
 mod boundary;
+mod globalvar;
 mod split;
 
+use self::globalvar::expand_global_addr;
+
 /// Legalize `func` for `isa`.
 ///
 /// - Transform any instructions that don't have a legal representation in `isa`.