Description:
 * AIX applications using 128-bit long double must be linked with
   libc128.a, in order to have 128-bit compatible routines.

Tests:
 * AIX 7.2, 7.1, 6.1: Build/Tests: OK

Changelog:
2020-04-03 Clément Chigot <clement.chigot@atos.net>
 * config/rs6000/aix61.h (LIB_SPEC): Add -lc128 with -mlong-double-128.
 * config/rs6000/aix71.h (LIB_SPEC): Likewise.
 * config/rs6000/aix72.h (LIB_SPEC): Likewise.