stdint.h: rearrange declarations into separate files

stdint.h: rearrange declarations into separate files

The current machine/stdint.h situation is a mess.  It declares only the
__versions of stdint integers, but also defines limit and constant
macros.  Furthermore it also declares non-integer types.

Break out integer limit and constant macros into their own machine/
headers and include all of these headers from <stdint.h>, but not from
machine/stdint.h.  The declaration of integer_t types still happens in
main stdint.h.

This is slightly based on NetBSD's way of doing it.