On 32 bit architectures define (u)int64_t as (unsigned) long long instead

On 32 bit architectures define (u)int64_t as (unsigned) long long instead
of (unsigned) int __attribute__((__mode__(__DI__))). This aligns better
with macros such as (U)INT64_C, (U)INT64_MAX, etc. which assume (u)int64_t
has type (unsigned) long long.

The mode attribute was used because long long wasn't standardised until
C99. Nowadays compilers should support long long and use of the mode
attribute is discouraged according to GCC Internals documentation.

The type definition has to be marked with __extension__ to support
compilation with "-std=c89 -pedantic".

Discussed with:	bde
Approved by:	kib (mentor)