Don't copy executable code into a CodeMemory (#3265)

* Don't copy executable code into a `CodeMemory`

This commit moves a copy from compiled artifacts into a `CodeMemory`. In
general this commit drastically changes the meaning of a `CodeMemory`.
Previously it was an iteratively-pushed-on structure that would
accumulate executable code over time. Afterwards, however, it's a
manager for an `MmapVec` which updates the permissions on text section
to ensure that the pages are executable.

By taking ownership of an `MmapVec` within a `CodeMemory` there's no
need to copy any data around, which means that the `.text` section in
the ELF image produced by Wasmtime is usable as-is after placement in
memory and relocations have been resolved. This moves Wasmtime one step
closer to being able to directly use a module after it's `mmap`'d into
memory, optimizing when a module is loaded.

* Fix windows section alignment

* Review comments

crates/cranelift/src/obj.rs

@@ -396,7 +396,7 @@ impl<'a> ObjectBuilder<'a> {
             unwind_info.extend_from_slice(&info.end.to_le_bytes());
             unwind_info.extend_from_slice(&info.unwind_address.to_le_bytes());
         }
-        self.obj.append_section_data(section_id, &unwind_info, 1);
+        self.obj.append_section_data(section_id, &unwind_info, 4);
     }
 
     /// This function appends a nonstandard section to the object which is only