rs6000: AIX long double builtins for 64 bit long double.

rs6000: AIX long double builtins for 64 bit long double.

When long doubles are 64 bit, the AIX C library overrides the definitions
but GCC builtins point to 128 bit names.  This patch overrides the
builtins for fmodl, frexpl, ldexpl and modfl to refer to the 64 bit symbols.

Backport from mainline
2020-05-04  Clement Chigot  <clement.chigot@atos.net>
            David Edelsohn  <dje.gcc@gmail.com>

        * config/rs6000/rs6000-call.c (rs6000_init_builtins): Override explicit
        for fmodl, frexpl, ldexpl and modfl builtins.