Add a MULTILIB option to gcc{,48,49,5} for powerpc64

This change is the same as r400632, which updated gcc[56]-devel, but now
for gcc{,48,49,5}. This change is the second attempt at doing this: the
first attempt went in r401072 and was reverted in r401074 because the diff
was bogus and enabled the new MULTILIB option under all platforms instead
of just powerpc64.

This fixes the build of gcc{,48,49,5} 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