Add -fmax-type-align=8 to CFLAGS when clang compiler is detected.

The pymalloc memory allocator aligns memory on 8 bytes. On x86-64,
clang expects alignment on 16 bytes by default and so uses MOVAPS
instruction which can lead to segmentation fault. Instruct clang that
Python is limited to alignemnt on 8 bytes to use MOVUPS instruction
instead: slower but don't trigger a SIGSEGV if the memory is not
aligned on 16 bytes.

Sadly, the flag must be expected to CFLAGS and not just
CFLAGS_NODIST, since third party C extensions can have the same
issue.

On AIX, sys.platform doesn't contain the major version anymore.
Always return 'aix', instead of 'aix3' .. 'aix7'.  Since
older Python versions include the version number, it is recommended to
always use sys.platform.startswith('aix').

"make tags" and "make TAGS" now also parse Modules/_io/*.c
and Modules/_io/*.h.

Use autoconfig to probe for shm_open() and shm_unlink().  Set SHM_NEEDS_LIBRT if we must
link with librt to get the shm_* functions.  Change setup.py to use the autoconfig defines.  These
changes should make it more likely that _multiprocessing/posixshmem.c gets built correctly on
different platforms.

Add a new os.posix_spawnp() function.