Removes duplicate code in src/obj.rs, crates/obj and crates/jit/object.rs (#1993)

Changes:

 -  Moves object creation code from crates/jit/object.rs to the creates/obj (as ObjectBuilder)
 -   Removes legacy crates/obj/function.rs
 -  Removes write_debugsections

crates/jit/src/link.rs

@@ -133,7 +133,7 @@ fn apply_reloc(
 
 fn to_libcall_address(name: &str) -> Option<usize> {
     use self::libcalls::*;
-    use crate::for_each_libcall;
+    use wasmtime_environ::for_each_libcall;
     macro_rules! add_libcall_symbol {
         [$(($libcall:ident, $export:ident)),*] => {
             Some(match name {