Auto-sync with upstream branch master
    
    Upstream commit: cfba5dbb10cc3abde632b46c60c10b2843917035
    
    - Keep expected behaviour for [a-z] and [A-z] (#1607286)
    - Additional ucontext tests
    - Intel CET enhancements
    - ISO C11 threads support
    - Fix out-of-bounds access in IBM-1390 converter (swbz#23448)
    - New locale Yakut (Sakha) for Russia (sah_RU) (swbz#22241)
    - os_RU: Add alternative month names (swbz#23140)
    - powerpc64: Always restore TOC on longjmp (swbz#21895)
    - dsb_DE locale: Fix syntax error and add tests (swbz#23208)
    - Improve performance of the generic strstr implementation
    - regcomp: Fix off-by-one bug in build_equiv_class (swbz#23396)
    - Fix out of bounds access in findidxwc (swbz#23442)