V3 PATCH: numeric_limits<> support, fix PR/3865

[Gabriel Dos Reis]

"Joseph S. Myers" <jsm28@cam.ac.uk> writes:

| Also, the _BITS__ macros for integer types are redundant with the _MAX__ 
| macros used for <limits.h>; one can be derived from the other.

Since we already define _BIT__ for char and similar macros are needed
for floating point types, I'll propose, for consistency to provide it
for the rest of the integer types.

-- Gaby