[3.14] gh-144981: Make PyUnstable_Code_SetExtra/GetExtra thread-safe (GH-144980)

[yoney]

• Make PyUnstable_Code_SetExtra/GetExtra thread-safe
  (cherry picked from commit a565327)

• Add the missing macros to fix the original backport [3.14] gh-144981: Make PyUnstable_Code_SetExtra/GetExtra thread-safe (GH-144980) #145052

cc: @DinoV@colesbury@mpage

• Issue: PyUnstable_Code_SetExtra and PyUnstable_Code_GetExtra are not thread-safe in free-threaded builds #144981

[colesbury]

I've cherry picked your change onto the Miss Islington PR:

• [3.14] gh-144981: Make PyUnstable_Code_SetExtra/GetExtra thread-safe (GH-144980) #145052