Auto merge of #60387 - Goirad:test-expansion, r=ollie27

Allow cross-compiling doctests

This PR allows doctest to receive a --runtool argument, as well as possibly many --runtool-arg arguments, which are then used to run cross compiled doctests.
Also, functionality has been added to rustdoc to allow it to skip testing doctests on a per-target basis, in the same way that compiletest does it. For example, tagging the doctest with "ignore-sgx" disables testing on any targets that contain "sgx". A plain "ignore" still skips testing on all targets.

See [here](https://github.com/rust-lang/cargo/pull/6892) for the companion PR in the cargo project that extends functionality in Cargo so that it passes the appropriate parameters to rustdoc when cross compiling and testing doctests.

Part of [#6460](https://github.com/rust-lang/cargo/issues/6460)

src/librustdoc/lib.rs

@@ -356,6 +356,23 @@ fn opts() -> Vec<RustcOptGroup> {
                       "show-coverage",
                       "calculate percentage of public items with documentation")
         }),
+        unstable("enable-per-target-ignores", |o| {
+            o.optflag("",
+                      "enable-per-target-ignores",
+                      "parse ignore-foo for ignoring doctests on a per-target basis")
+        }),
+        unstable("runtool", |o| {
+            o.optopt("",
+                     "runtool",
+                     "",
+                     "The tool to run tests with when building for a different target than host")
+        }),
+        unstable("runtool-arg", |o| {
+            o.optmulti("",
+                       "runtool-arg",
+                       "",
+                       "One (of possibly many) arguments to pass to the runtool")
+        }),
     ]
 }