Stabilize flags for doctest cross compilation

[ehuss]

This makes the following changes in preparation for supporting doctest cross-compiling in cargo:

• Renames --runtool and --runtool-arg to --test-runtool and --test-runtool-arg to maintain consistency with other --test-* arguments.
• Stabilizes the --test-runtool and --test-runtool-arg. These are needed in order to support cargo's target.runner option which specifies a runner to execute a cross-compiled doctest (for example, qemu).
• Stabilizes the --enable-per-target-ignores flag by removing it and making it unconditionally enabled. This makes it possible to disable a doctest on a per-target basis, which I think will be helpful for rolling out this feature.

These changes were suggested in https://rust-lang.zulipchat.com/#narrow/channel/266220-t-rustdoc/topic/stabilizing.20doctest.20xcompile/near/409281127

The intent is to stabilize the doctest-xcompile feature in cargo. This will help ensure that for projects that do cross-compile testing that their doctests are also covered. Currently there is a somewhat surprising behavior that they are ignored.

Closes #64245

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[rustbot]

This PR modifies tests/run-make/. If this PR is trying to port a Makefile
run-make test to use rmake.rs, please update the
run-make port tracking issue
so we can track our progress. You can either modify the tracking issue
directly, or you can comment on the tracking issue and link this PR.

cc @jieyouxu

The Miri subtree was changed

cc @rust-lang/miri