Partially MFC r209867 and r215439, to provide rudimentary support for

Partially MFC r209867 and r215439, to provide rudimentary support for
linking i386 (32 bit) executables on amd64, using gcc's -m32 option.

r209867 introduces the needed multilib options to gcc's multilib.h, but
was originally intended just for powerpc64.

r215439 teaches the linker to search in /usr/lib32 for elf_i386_fbsd
output, and amends r209867 to apply to amd64 too.

NOTE: this MFC does NOT provide full support for compiling and linking
all existing 32 bit programs on amd64, since /usr/include/machine still
contains amd64-specific headers.  Though simple "Hello World" type
programs work, anything relying on machine-specific types might fail in
various interesting ways.  You have been warned. :)

MFC r209867:
Teach our toolchain how to generate 64-bit PowerPC binaries. This fixes
a variety of bugs in binutils related to handling of 64-bit PPC ELF,
provides a GCC configuration for 64-bit PowerPC on FreeBSD, and
associated build systems tweaks.

Obtained from:	projects/ppc64

MFC r215439:
Let gcc and ld know where to find 32 bit libraries on amd64.

Reviewed by:	arch@
Approved by:	kib (mentor)

git-svn-id: svn://svn.freebsd.org/base/stable/8@219648 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f