Auto-sync with upstream branch master
    
    Upstream commit: 1305edd42c44fee6f8660734d2dfa4911ec755d6
    
    - elf: Move post-relocation code of _dl_start into _dl_start_final
    - misc: Fix rare fortify crash on wchar funcs. [BZ 29030]
    - elf: Remove unused enum allowmask
    - scripts/glibcelf.py: Mark as UNSUPPORTED on Python 3.5 and earlier
    - x86: Optimize {str|wcs}rchr-evex
    - x86: Optimize {str|wcs}rchr-avx2
    - x86: Optimize {str|wcs}rchr-sse2
    - benchtests: Improve bench-strrchr
    - x86-64: Fix SSE2 memcmp and SSSE3 memmove for x32
    - Default to --with-default-link=no (bug 25812)
    - scripts: Add glibcelf.py module
    - Add locale for syr_SY
    - elf: Move elf_dynamic_do_Rel RTLD_BOOTSTRAP branches outside
    - m68k: Handle fewer relocations for RTLD_BOOTSTRAP (#BZ29071)
    - nptl: Fix pthread_cancel cancelhandling atomic operations
    - x86: Fix missing __wmemcmp def for disable-multiarch build
    - elf: Remove __libc_init_secure