Add a MULTILIB option to gcc[56]-devel for powerpc64


Add a MULTILIB option to gcc[56]-devel for powerpc64

This fixes the build of gcc[56]-devel under powerpc64 when the system is
built without the lib32 libraries.

More in detail:

If the system is built with lib32 support (WITH_LIB32, which is the default),
building gcc from ports results in a compiler that can target both 64-bit and
32-bit binaries on powerpc64. However, when lib32 support is disabled
(WITHOUT_LIB32), gcc should only be built with 64-bit support or otherwise
the build fails.

To fix this, explicitly disable 32-bit support when /usr/lib32 is not present
and add a MULTILIB option (which is only defined for powerpc64 when 32-bit
support is possible and defaults to yes to preserve the current behavior) to
allow the user to explicitly control this feature.

Approved by: gerald (maintainer), bdrewery (mentor), andreast
Differential Revision: https://reviews.freebsd.org/D3952