move limits for sNN/uNN types from kernel.h to limits.h

move limits for sNN/uNN types from kernel.h to limits.h

Since we define the {s,u}{8,16,32,64} types the same way on all
architectures, i.e. everybody uses asm-generic/int-ll64.h, we can just
define the associated limit macros in terms of those for the
corresponding types. This eliminates another set of limit macros that
are not usable in #if conditionals.

These type names and macros are not C or POSIX, so there's no language
violation, but certainly a violation of developers' reasonable
expectations.

Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>