[3.12] gh-110429: Fix race condition in "make regen-all" (GH-110433) (#110438)

gh-110429: Fix race condition in "make regen-all" (GH-110433)

"make regen-pegen" now creates a temporary file called "parser.c.new"
instead of "parser.new.c". Previously, if "make clinic" was run in
parallel with "make regen-all", clinic may try but fail to open
"parser.new.c" if the temporay file was removed in the meanwhile.
(cherry picked from commit fb6c4ed2bbb2a867d5f0b9a94656e4714be5d9c2)

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