aix: Switch AIX configurtion to DWARF2 debugging
This patch is in preparation for removing stabs debugging support from GCC.
The rs6000 configuration files remain somewhat intertwined with the
stabs debugging support, but the configuration no longer generates
stabs debugging information.
This patch means that earlier releases (Technology Levels) of AIX 7.1
and 7.2, prior to DWARF support and fixes, cannot build GCC or support
GCC.
gcc/ChangeLog:
* config/rs6000/aix71.h (PREFERRED_DEBUGGING_TYPE): Change to
DWARF2_DEBUG.
* config/rs6000/aix72.h (PREFERRED_DEBUGGING_TYPE): Same.