doc: add the multi-target porting checklist

Ports opt into multi-target builds by declaring multi_target_ready in
config.gcc, and the declaration is a promise about the port's shape: prefixed
exports, headers that compile in a captured context, macro values that no shared
layout depends on, descriptor coverage for anything host code reads. The manual
now walks a port maintainer through that checklist, each item tied to the
mechanism that depends on it and to the identity gates that certify the result.
The blob contract closes the chapter: what a blob may export and what its fields
may reference, and how that narrowness lets the producer move from N generator
runs with renames to merged target-indexed tables one generator at a time,
without consumers or the registry ever changing.

gcc/ChangeLog:

	* doc/multi-target.texi: Add the porting checklist and the producer
	contract.