make size_t better approximate the reality

make size_t better approximate the reality

Instead of "always unsigned long" go for "unsigned int unless
-m64 is given, unsigned long otherwise".  Add an option (-msize-long)
forcing to unsigned long regardless.  Make __SIZE_TYPE__ expansion
match that.

The thing is, addition of checks on comparisons make for very unhappy
min() on (kernel) size_t and sizeof(something) on the targets where
the former is unsigned int.  Which is to say, more than half of them...
AFAICS, the only place needing explicit -msize-long in CHECK_FLAGS
is s390 (it's using unsigned long both for 31- and 64-bit).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>