Merge #1003

1003: Add more intrinsics and refactor how we implement them r=philberty a=philberty

This patch series implements:

1. offset
2. size_of
3. unreachable
4. abort

It removes the GCC wrapper mappings to make them much easier to implement. It also demonstrates in single commits
the implementation of each of these intrinsic to make it easy to follow in how we implement them.

Addresses #658 #849 

Co-authored-by: Philip Herron <philip.herron@embecosm.com>

gcc/rust/backend/rust-compile-fnparam.h

@@ -36,6 +36,14 @@ class CompileFnParam : public HIRCompileBase, public HIR::HIRPatternVisitor
     return compiler.compiled_param;
   }
 
+  static Bvariable *compile (Context *ctx, tree fndecl, HIR::Pattern *param,
+			     tree decl_type, Location locus)
+  {
+    CompileFnParam compiler (ctx, fndecl, decl_type, locus);
+    param->accept_vis (compiler);
+    return compiler.compiled_param;
+  }
+
   void visit (HIR::IdentifierPattern &pattern) override
   {
     if (!pattern.is_mut ())