rustc_target: Rename some target options to avoid tautology

`target.target_endian` -> `target.endian`
`target.target_c_int_width` -> `target.c_int_width`
`target.target_os` -> `target.os`
`target.target_env` -> `target.env`
`target.target_vendor` -> `target.vendor`
`target.target_family` -> `target.os_family`
`target.target_mcount` -> `target.mcount`

compiler/rustc_target/src/spec/openbsd_base.rs

@@ -16,10 +16,10 @@ pub fn opts() -> TargetOptions {
     );
 
     TargetOptions {
-        target_os: "openbsd".to_string(),
+        os: "openbsd".to_string(),
         dynamic_linking: true,
         executables: true,
-        target_family: Some("unix".to_string()),
+        os_family: Some("unix".to_string()),
         linker_is_gnu: true,
         has_rpath: true,
         abi_return_struct_as_int: true,