Add special psuedo-options for the base/{binutils,gcc} ports.

The WITH_PORT_BASE_{BINUTILS,GCC} options are used to prevent 'make check-old'
and 'make delete-old' from deleting files installed by the base/binutils
and base/gcc packages as normally one disables the in-tree variants
(e.g. WITHOUT_BINUTILS) when using these packages.

Reviewed by: bapt (earlier version)
Differential Revision: https://reviews.freebsd.org/D19483