Auto-sync with upstream master
    
    Upstream commit: 2585d7b839559e665d5723734862fbe62264b25d
    
    - Do not use generic selection in C++ mode
    - Do not use __builtin_types_compatible_p in C++ mode (#1481205)
    - x86-64: Check FMA_Usable in ifunc-mathvec-avx2.h (swbz#21966)
    - Various locale fixes (swbz#21750, swbz#21960, swbz#21959, swbz#19852)
    - Fix sigval namespace (swbz#21944)
    - x86-64: Optimize e_expf with FMA (swbz#21912)
    - Adjust glibc-rh827510.patch.