Now that GCC 5 has branched for the release of GCC 5.1, create a new port
gcc6-devel to track mainline GCC development, starting with the 20150412
snapshot of GCC 6.0.0.
(Since this really is a development version not necessarily recommended
for general use, start using the -devel moniker for this port. This is
not due to any changes in upstream policy or expected attributes of this
port, only an adjustment in how we name things.)