This port, lang/gcc48, is only still alive due to dependencies by
sysutils/uefi-edk2-bhyve and sysutils/memtest86+. Its successors
lang/gcc49 and lang/gcc5 have left the tree already, and lang/gcc6
is scheduled for removal soon.
Thus at least restrict to i386 and amd64, the only two platforms
supported by those two dependant ports.