Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Use __builtin_va_start instead of __builtin_stdarg_start. GCC4 obsoletes
the former and __builtin_va_start was present in all GCC version 3.1 and
later.

netchild's mega-patch to isolate compiler dependencies into a central
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

In the spirit of amd64/include/stdarg.h rev 1.6; add __va_copy
(but keep it conditional on __ISO_C_VISIBLE >= 1999.

Why? Our out /usr/src/contrib assumes it, and more than a few ports have
an autoconf that looks for __va_copy because it is available on glibc.
It is critical that we use it on PowerPC. It generally isn't a problem
for i386 and its ilk because those platforms can get away with cheating
the C standard, using a plain assignment.

o Add conditionals to allow va_list to be defined in other headers.
o Standardize on _MACHINE_STDARG_H_ to allow multiple header includes.
o Restrict the definition of va_copy() to C99 environments.

Implement C99's va_copy() macro.

o Merge <machine/ansi.h> and <machine/types.h> into a new header
called <machine/_types.h>.
o <machine/ansi.h> will continue to live so it can define MD clock
macros, which are only MD because of gratuitous differences between
architectures.
o Change all headers to make use of this. This mainly involves
changing:
#ifdef _BSD_FOO_T_
typedef _BSD_FOO_T_ foo_t;
#undef _BSD_FOO_T_
#endif
to:
#ifndef _FOO_T_DECLARED
typedef __foo_t foo_t;
#define _FOO_T_DECLARED
#endif

Concept by: bde
Reviewed by: jake, obrien

Gcc 3.1 varargs support.

Cleanup of the stdarg code.

Submitted by: ru

Add support for the gcc-2.95 stdarg implementation.

Bring in NetBSD code used in the PowerPC port.

Reviewed by: obrien, dfr
Obtained from: NetBSD