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

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

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.