I did "emerge =dev-lang/python-2.7.5-r1" *twice* with the environment configuration described in msg196520, then *once* with it disabled because one of the cases you described was when you tried to revert to a non-debug Python.  (Besides, I am willing to risk your crash while I'm watching for it, but not a time bomb that will go off when I'm on deadline :-).  All builds succeeded and all passed the test suite.  Here's how the debug build describes itself:

== CPython 2.7.5 (default, Sep 1 2013, 00:59:02) [GCC 4.6.4] 
==   Linux-3.9.0-x86_64-Dual_Core_AMD_Opteron-tm-_Processor_265-with-gentoo-2.2 little-endian 

The test suite ran uneventfully (with a few DeprecationWarnings) except for this:

6 skips unexpected on linux2: 
    test_bsddb test_bsddb3 test_tcl test_tk test_ttk_guionly 
    test_ttk_textonly 

but I suppose that is expected on Gentoo.  If any of those modules (bsddb, tcl, tk) are built into your Python, a problem in one of those might be the culprit.

Oh, damn.  I just reread the whole thread.  For some reason I thought you were using gcc 4.6.4, but now I see you report 4.7.3.  OK build with 4.7.3 and your flags (also restore the --with-pydebug config):

# export CFLAGS=" -ggdb -pipe -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx -maes -mpclmul -mpopcnt"
# export CXXFLAGS="${CFLAGS}"
# export CC=gcc-4.7.3

and we crash (from make output) immediately after linking ./python:

x86_64-pc-linux-gnu-ranlib libpython2.7.a 
gcc-4.7.3 -pthread -Wl,--hash-style=gnu -Wl,-O1 -Wl,--as-needed -L. -Xlinker -export-dynamic -o python \ 
                Modules/python.o \ 
                -L. -lpython2.7 -lpthread -ldl  -lutil   -lm   
LD_LIBRARY_PATH=/var/tmp/portage/dev-lang/python-2.7.5-r1/work/x86_64-pc-linux-gnu: ./python -E -S -m sysconfig --generate-posix-vars 
make: *** [pybuilddir.txt] Illegal instruction 

However, I'm pretty sure this is due to my hardware not liking your -m flags, not the crash you reported.  I'll try backing those flags out, but if anybody has a suggestion for the most aggressive set similar to yours, I'd appreciate it.

But first this process is going to go sleep(25200).