Dpkg::Vendor::Debian: Improve PIE flags support

Fix changelog for dpkg 1.18.11 to mention PIE got enabled by default for
all architectures, not just the ones where gcc does that itself.

When emitting PIE flags on architectures where gcc does not inject those
itself, do it via a specs file too, so that maintainers can use them
unconditionally regardless of the object being compiled or linked.

When injecting -no-pie for linking via gcc specs also inject -fno-PIE.

Update the documentation to make the current situation more clear.