merge revision(s) 63abb5c2,34d02631,2adbf01a,3acc81d9: [Backport #17612]

	dtoa.c: make compilable independently

	Except for `-Dxmalloc=malloc -Dxfree=free`.
	---
	 missing/dtoa.c | 24 ++++++++++++++++++------
	 1 file changed, 18 insertions(+), 6 deletions(-)

	dtoa.c: constified

	clang seems to locate never modified local data in the const
	segment implicitly.
	---
	 missing/dtoa.c | 2 +-
	 1 file changed, 1 insertion(+), 1 deletion(-)

	dtoa.c: make thread-safe by using atomic CAS

	---
	 common.mk      |  2 ++
	 missing/dtoa.c | 75 ++++++++++++++++++++++++++++++++++++++++++----------------
	 util.c         |  1 +
	 3 files changed, 57 insertions(+), 21 deletions(-)

	Fixed race in dtoa [Bug #17612]

	Fixed the race condition when replacing `freelist` entry with its
	chained next element.  At acquiring an entry, hold the entry once
	with the special value, then release by replacing it with the next
	element again after acquired.  If another thread is holding the
	same entry at that time, spinning until the entry gets released.

Co-Authored-By: Koichi Sasada <ko1@atdot.net>
	---
	 bootstraptest/test_ractor.rb | 11 +++++++++++
	 missing/dtoa.c               | 13 ++++++++++---
	 2 files changed, 21 insertions(+), 3 deletions(-)