MFC r232262 by tijl:

  Copy amd64 _limits.h to x86 and merge with i386 _limits.h. Replace
  amd64/i386/pc98 _limits.h with stubs.

MFC (part of) r235939 by obrien:

  Consitently use "__LP64__".

Sponsored by:	The FreeBSD Foundation
MFC r232264 by tijl:

  Copy amd64 _stdint.h to x86 and merge with i386 _stdint.h. Replace
  amd64/i386/pc98 _stdint.h with stubs.

MFC r232519 by tijl:

  Do not use INT64_C and UINT64_C to define 64 bit integer limits.  They
  aren't defined for C++ code unless __STDC_CONSTANT_MACROS is defined.

  Reported by:      jhb

MFC (part of) r235939 by obrien:

  Consitently use "__LP64__".
  [there are 33 __LP64__'s in the kernel (minus cddl/ and contrib/),
  and 11 _LP64's]

Sponsored by:	The FreeBSD Foundation
Document r260909.

Sponsored by:	The FreeBSD Foundation
Document r260891.

Sponsored by:	The FreeBSD Foundation
Document r260868, r260869.

Sponsored by:	The FreeBSD Foundation