Add sys.float_repr_style attribute ('short' if short float
repr is in used;  'legacy' otherwise).

 - add double endianness detection to configure script
 - add configure-time check to see whether we can use inline
   assembly to get and set x87 control word in configure script
 - add functions to get and set x87 control word in Python/pymath.c
 - add pyport.h logic to determine whether it's safe to use the
   short float repr or not

Add the Python/dtoa.c file containing the main algorithms;
add corresponding include file and include in Python.h;
include license information for Python/dtoa.c;
add dtoa.c and dtoa.h to Makefile.

Per the discussion in issue6882, backport the try/finally work that was done to the py3k version (mostly in r59477, I think).