Push a real iseq in rb_vm_push_frame_fname()

	Previously, vm_make_env_each() (used during proc
	creation and for the debug inspector C API) picked up the
	non-GC-allocated iseq that rb_vm_push_frame_fname() creates,
	which led to a SEGV when the GC tried to mark the non GC object.

	Put a real iseq imemo instead. Speed should be about the same since
	the old code also did a imemo allocation and a malloc allocation.

	Real iseq allows ironing out the special-casing of dummy frames in
	rb_execution_context_mark() and rb_execution_context_update(). A check
	is added to RubyVM::ISeq#eval, though, to stop attempts to run dummy
	iseqs.

	[Bug #21180]

	Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

	Ensure fiber scheduler re-acquires mutex when interrupted from sleep. (#12158)

	[Bug #20907]

`ppc64le` appears to be struggling with this test due to timeout. Let's see
if reducing the number of iterations can help improve the test performance.

* Ensure that blocked fibers don't prevent valid wakeups.

to prevent the following failure on `make test-all --repeat-count=2`

http://ci.rvm.jp/results/trunk-repeat20-asserts@phosphorus-docker/3957774
```
  1) Error:
TestFiberScheduler#test_autoload:
NameError: uninitialized constant TestFiberSchedulerAutoload
          Object.const_get(:TestFiberSchedulerAutoload)
                ^^^^^^^^^^
```

Object#autoload implements a custom per-thread "mutex" for blocking
threads waiting on autoloading a feature. This causes problems when used
with the fiber scheduler. We swap the implementation to use a Ruby mutex
which is fiber aware.