Add TLS support for ELF and MachO (#1174)

* Add TLS support
* Add binemit and legalize tests
* Spill all caller-saved registers when necessary

cranelift/codegen/meta/src/shared/mod.rs

@@ -1,6 +1,6 @@
 //! Shared definitions for the Cranelift intermediate language.
 
-mod entities;
+pub mod entities;
 pub mod formats;
 pub mod immediates;
 pub mod instructions;
@@ -28,6 +28,7 @@ pub(crate) struct Definitions {
     pub imm: Immediates,
     pub formats: Formats,
     pub transform_groups: TransformGroups,
+    pub entities: EntityRefs,
 }
 
 pub(crate) fn define() -> Definitions {
@@ -47,6 +48,7 @@ pub(crate) fn define() -> Definitions {
         imm: immediates,
         formats,
         transform_groups,
+        entities,
     }
 }