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}. Waited a week to ensure the change caused nothing to go
horribly wrong but this change is very low risk because it only affects
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