Introduce multiarch DISTRO_FEATURE

This patch introduces a distro feature which enables gcc to produce
both 32bit and 64bit code, and enables binutils to operate on both
32bit and 64bit binaries. It differs from multilib toolchains in
that it does not require to compile a version of the libc for each
architecture variant. However, the code produced for the secondary
architecture will not be linkable against the libc.

v2: - Renamed the feature name from "biarch" to "multiarch". The GCC
installation manual claims that the mips-linux can be made a tri-arch
compiler (http://gcc.gnu.org/install/configure.html)
    - For x86_64, the compiler is made bi-arch by default, so nothing
has to be done in particular.
    - I analyzed the gcc/config.gcc from GCC sources and added in this
patch all the architectures that could be made biarch with the version
of gcc currently used in OE, which are powerpc, and sparc, in addition
to x86. mips and s390 will probably be supported in future versions of
gcc. For x86 and sparc, only the --enable-targets=all option is valid
to make this work (this option doesn't have any other side effects than
making the compiler bi-arch). For powerpc, I used the
--enable-targets=powerpc64 option (although 'all' also works).

Note: - Untested on powerpc and sparc. But I believe it works the same
as with x86.
      - gcc in meta-toolchain is also made multiarch.

Signed-off-by: Julian Pidancet <julian.pidancet@gmail.com>

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