Add WITH_BASE_BINUTILS and WITH_BASE_GCC knobs for check-old/delete-old.

[jhb]

Summary

I'm definitely open to suggestions if we don't like the names. The idea
is to let base/binutils set WITHOUT_BINUTILS=yes but then not have
'make delete-old' delete binaries installed by the base/binutils
package. Brandon suggested the slightly more verbose WITH_PORT_BASE_BINUTILS.
Note that users wouldn't set these directly in src.conf. Instead, the
files base/* install to /usr/share/toolchains would set these knobs.

Test Plan

• make delete-old in an amd64 VM using base/* as the toolchain and then use 'pkg check' to ensure nothing is missing