Implement the basics of the x86-64 ABI.

This is just a rough sketch to get us started. There are bound to be
some issues.

This also legalizes signatures for x86-32, but probably not correctly.
It's basically implementing the x86-64 ABI for 32-bit.

lib/cretonne/src/isa/intel/mod.rs

@@ -88,7 +88,7 @@ impl TargetIsa for Isa {
     }
 
     fn allocatable_registers(&self, func: &ir::Function) -> regalloc::AllocatableSet {
-        abi::allocatable_registers(func)
+        abi::allocatable_registers(func, &self.shared_flags)
     }
 
     fn emit_inst(&self, func: &ir::Function, inst: ir::Inst, sink: &mut CodeSink) {