std_limits.h os_defines.h

[Benjamin Kosnik]

I think this patch is pretty weak. Some of the targets use LONG_MAX, some 
use stuff like __alpha, etc. I don't see any reason to not use LONG_MAX, 
as was discussed.

I would prefer it if all the bits for std_limits.h went in together, so 
it's obvious what is going on.

-benjamin