64f508ade8 changed rb_threadptr_raise to call rb_exception_setup,
which uses rb_scan_args with RB_SCAN_ARGS_PASS_CALLED_KEYWORDS.
This checked rb_keyword_given_p(), which read the interrupted
frame's keyword state rather than the signal raise arguments,
causing a crash when a signal arrived during a keyword call.

Revert rb_threadptr_raise to use rb_make_exception directly, and
have thread_raise_m call rb_exception_setup where
rb_keyword_given_p() reflects the correct frame.

[Bug #21865]

This test was relying on a sleep to synchronise the parent and child
processes. By having the child be the process that stops itself with
SIGSTOP, instead of the parent, we can actually properly wait for that
using waitpid2 and be notified of the stop.

This use of sleep to synchronise processes is potentially flaky and
caused failures under rr's `--chaos` mode.

* Remove `waitpid_lock` and related code.

* Remove un-necessary test.

* Remove `rb_thread_sleep_interruptible` dead code.

* Revert "Remove special handling of `SIGCHLD`. (#7482)"

This reverts commit 44a0711eab7fbc71ac2c8ff489d8c53e97a8fe75.

* Revert "Remove prototypes for functions that are no longer used. (#7497)"

This reverts commit 4dce12bead3bfd91fd80b5e7195f7f540ffffacb.

* Revert "Remove SIGCHLD `waidpid`. (#7476)"

This reverts commit 1658e7d96696a656d9bd0a0c84c82cde86914ba2.

* Fix change to rjit variable name.

* Remove `waitpid_lock` and related code.

* Remove un-necessary test.

* Remove `rb_thread_sleep_interruptible` dead code.

Two tests abort intentionally and they create core files if
possible. In these case, we don't need to see core files
so disable by `"Process.setrlimit(Process::RLIMIT_CORE, 0)` for
those cases.

This removes the related tests, and puts the related specs behind
version guards.  This affects all code in lib, including some
libraries that may want to support older versions of Ruby.