merge revision(s) 86c26254,7e147621: [Backport #17935]

	Fix a race condition around mjit_recompile

	This fixes SEGVs like https://github.com/ruby/ruby/runs/2715166621?check_suite_focus=true.

	When mjit_recompile is called when mjit_compile is compiling the exact
	same iseq (and after it called mjit_capture_cc_entries), iseq->body->jit_unit
	is re-created and its cc_entries becomes NULL. Then, when it tries to
	lookup cc_entries through iseq->body->jit_unit, it fails.
	---
	 mjit.c        | 21 +++++++++++++--------
	 mjit_worker.c |  4 ++++
	 2 files changed, 17 insertions(+), 8 deletions(-)

	Do not doubly hold an MJIT lock

	This is a follow-up of 86c26254.
	CRITICAL_SECTION_START/FINISH are not needed when it's called from an
	MJIT worker.

	Also, ZALLOC needs to be calloc because ZALLOC may trigger GC, which an
	MJIT worker must not do.
	---
	 mjit.c        | 23 ++++++++++++++---------
	 mjit_worker.c |  4 ++--
	 2 files changed, 16 insertions(+), 11 deletions(-)