restore the stack pointer on finalizer

	When error on finalizer, the exception will be ignored.
	To restart the code, we need to restore the stack pointer.

	fix [Bug #20042]
	---
	 gc.c                 | 4 ++++
	 test/ruby/test_gc.rb | 9 +++++++++
	 2 files changed, 13 insertions(+)

The test added in 90a80eb0 fails if Ruby is verbose, it outputs the
following line to stderr:

    RUBY_GC_HEAP_INIT_SLOTS=100 (default value: 10000)

	Fix integer underflow when using HEAP_INIT_SLOTS

	There is an integer underflow when the environment variable
	RUBY_GC_HEAP_INIT_SLOTS is less than the number of slots currently
	in the Ruby heap.

	[Bug #19284]
	---
	 gc.c                 | 25 +++++++++++++------------
	 test/ruby/test_gc.rb |  5 +++++
	 2 files changed, 18 insertions(+), 12 deletions(-)

This commit adds RVALUE_OVERHEAD for storing metadata at the end of the
slot. This commit moves the ractor_belonging_id in debug builds from the
flags to RVALUE_OVERHEAD which frees the 16 bits in the headers for
object shapes.

The test could be flaky when a heap has below GC_HEAP_INIT_SLOTS number
of free slots because it would trigger a major GC and allocate more
pages.

The test is failing only on trunk-repeat50@phosphorus-docker. This
commit adds some debugging output to debug the failure.

GC could be in an intermediate state after creating the objects, so we
should finish GC by running a minor GC.

This test will prevent performance regressions like [Bug #18929].