winch: Small clean-up for x64 (#5691)

This commit contains a small set of clean up items for x64.

Notably:

* Adds filetests
* Documents why 16 for the arg base offset abi implementation, for clarity.
* Fixes a bug in the spill implementation caught while anlyzing the
filetests results. The fix consists of emitting a load instead of a store into
the scratch register before spiiling its value.
* Remove dead code for pretty printing registers which is not needed anymore
since we now have proper disassembly.

winch/codegen/src/regalloc.rs

@@ -134,9 +134,7 @@ impl RegAlloc {
                     .get_local(*index)
                     .expect("valid local at slot");
                 let addr = context.masm.local_address(&slot);
-                context
-                    .masm
-                    .store(RegImm::reg(self.scratch), addr, slot.ty.into());
+                context.masm.load(addr, self.scratch, slot.ty.into());
                 let offset = context.masm.push(self.scratch);
                 *v = Val::Memory(offset);
             }