[3.14] gh-130396: Move PYOS_LOG2_STACK_MARGIN to internal headers (GH-135928) (#136173)

gh-130396: Move PYOS_LOG2_STACK_MARGIN to internal headers (GH-135928)

Move PYOS_LOG2_STACK_MARGIN, PYOS_STACK_MARGIN,
PYOS_STACK_MARGIN_BYTES and PYOS_STACK_MARGIN_SHIFT macros to
pycore_pythonrun.h internal header. Add underscore (_) prefix to the
names to make them private. Rename _PYOS to _PyOS.
(cherry picked from commit 28940e8e4884eb3444a8ea15a5634f76b154d7ed)

Co-authored-by: Victor Stinner <vstinner@python.org>