re PR c++/54601 (AIX uses atexit which causes unloading of shared modules to break)

        PR target/54601
libgcc/
        * config.host (powerpc-ibm-aix[56789]): Add t-aix-cxa to tmake_file.
        Add crtcxa to extra_parts.
        * config/rs6000/exit.h: New file.
        * config/rs6000/cxa_atexit.c: New file.
        * config/rs6000/cxa_finalize.c: New file.
        * config/rs6000/crtcxa.c: New file.
        * config/rs6000/t-aix-cxa: New file.
        * config/rs6000/libgcc-aix-cxa.ver: New file.

gcc/
        * configure.ac (cxa_atexit): Add AIX.
        * configure: Regenerate.

        * config/rs6000/aix61.h (STARTFILE_SPEC): Add crtcxa.o.

From-SVN: r195675