place.

This moves the dependency on GCC's and other compiler's features into
the central sys/cdefs.h file, while the individual source files can
then refer to #ifdef __COMPILER_FEATURE_FOO where they by now used to
refer to #if __GNUC__ > 3.1415 && __BARC__ <= 42.

By now, GCC and ICC (the Intel compiler) have been actively tested on
IA32 platforms by netchild.  Extension to other compilers is supposed
to be possible, of course.

Submitted by:	netchild
Reviewed by:	various developers on arch@, some time ago

Approved by: core

 - Fix visibilty test for LONG_BIT and WORD_BIT.  `#if defined(__FOO_VISIBLE)'
   is alays wrong because __FOO_VISIBLE is always defined (to 0 for
   invisibility).

sys/<arch>/include/limits.h
sys/<arch>/include/_limits.h:

 - Style fixes.

Submitted by:	bde
Reviewed by:	bsdmike
Approved by:	re (scottl)

Change all in-tree consumers to include <sys/limits.h>

Discussed on:	standards@
Partially submitted by: Craig Rodrigues <rodrigc@attbi.com>

<stdint.h>.  Previously, parts were defined in <machine/ansi.h> and
<machine/limits.h>.  This resulted in two problems:
  (1) Defining macros in <machine/ansi.h> gets in the way of that
      header only defining types.
  (2) Defining C99 limits in <machine/limits.h> adds pollution to
      <limits.h>.

o Make <stdint.h> a symbolic link to <sys/stdint.h>.
o Move most of <sys/inttypes.h> into <sys/stdint.h>, as per C99.
o Remove <sys/inttypes.h>.
o Adjust includes in sys/types.h and boot/efi/include/ia64/efibind.h
  to reflect new location of integer types in <sys/stdint.h>.
o Remove previously symbolicly linked <inttypes.h>, instead create a
  new file.
o Add MD headers <machine/_inttypes.h> from NetBSD.
o Include <sys/stdint.h> in <inttypes.h>, as required by C99; and
  include <machine/_inttypes.h> in <inttypes.h>, to fill in the
  remaining requirements for <inttypes.h>.
o Add additional integer types in <machine/ansi.h> and
  <machine/limits.h> which are included via <sys/stdint.h>.

Partially obtain from:	NetBSD
Tested on:		alpha, i386
Discussed on:		freebsd-standards@bostonradio.org
Reviewed by:		bde, fenner, obrien, wollman