Update doctest xcompile flags

This updates the flags used for doctest xcompile to match the upstream
changes in rust-lang/rust#137096 which renamed
the flag.

Author: ehuss

src/cargo/ops/cargo_test.rs

@@ -238,12 +238,10 @@ fn run_doc_tests(
         }
 
         if doctest_xcompile {
-            p.arg("-Zunstable-options");
-            p.arg("--enable-per-target-ignores");
             if let Some((runtool, runtool_args)) = compilation.target_runner(unit.kind) {
-                p.arg("--runtool").arg(runtool);
+                p.arg("--test-runtool").arg(runtool);
                 for arg in runtool_args {
-                    p.arg("--runtool-arg").arg(arg);
+                    p.arg("--test-runtool-arg").arg(arg);
                 }
             }
             if let Some(linker) = linker {

src/doc/src/reference/unstable.md

@@ -285,8 +285,7 @@ This flag changes `cargo test`'s behavior when handling doctests when
 a target is passed. Currently, if a target is passed that is different
 from the host cargo will simply skip testing doctests. If this flag is
 present, cargo will continue as normal, passing the tests to doctest,
-while also passing it a `--target` option, as well as enabling
-`-Zunstable-features --enable-per-target-ignores` and passing along
+while also passing it a `--target` option, as well as passing along
 information from `.cargo/config.toml`. See the rustc issue for more information.
 
 ```sh