Fix AIX port.

Fix AIX port.

* configure.ac (DATA_START, DATA_SEG_BITS): Set to 0x20000000 on AIX.
(GC_MARK_STACK): Do not set to GC_USE_GCPROS_AS_BEFORE, as that
runs afoul of some other bug in Emacs, and the default value
GC_MAKE_GCPROS_NOOPS has been tested and works.
* src/lisp.h (XPNTR) [!USE_LSB_TAG && DATA_SEG_BITS]:
Fix bug introduced in 2012-07-27 change.  DATA_SEG_BITS, if set,
was #undeffed earlier, so it cannot be used as a macro here.
Use the constant and not the macro.  Tested on AIX.
* src/unexaix.c: Revert 2013-02-11 and 2013-02-12 changes to this
file.  They're almost surely OK but we're just before a release so
we should avoid changes unless they're clearly needed.  Instead,
make the following minor change:
(ADDR_CORRECT): New macro.