Auto-sync with upstream branch master
    
    Upstream commit: 2444ce5421c6036a503842d8dd8d93c27aad59ee
    
    - mach lll_lock/unlock: Explicitly request private locking
    - elf: Replace most uses of THREAD_GSCOPE_IN_TCB
    - Add MADV_POPULATE_READ and MADV_POPULATE_WRITE from Linux 5.14 to
      bits/mman-linux.h
    - Update kernel version to 5.14 in tst-mman-consts.py
    - configure: Fix check for INSERT in linker script
    - iconvconfig: Fix behaviour with --prefix [BZ #28199]
    - nptl: Fix race between pthread_kill and thread exit (bug 12889)
    - nptl: pthread_kill, pthread_cancel should not fail after exit (bug 19193)
    - benchtests: Remove redundant assert.h
    - benchtests: Enable scripts/plot_strings.py to read stdin
    - Add narrowing square root functions
    - _Static_assert needs two arguments for compatibility with GCC before 9
    - testrun.sh: Add support for --tool=rpctrace