Followup: Add release notes about ghc-toolchain

In !9263 (merged), for #19877 (closed), we created a tool called ghc-toolchain to define and configure Target description files to more easily configure multiple targets, and simplify the handling of multiple toolchains in the build system, but we have yet to add this to the release notes.

Here's a draft:

With the longer-term goal of runtime retargetability and multiple toolchain selection, we've introduced a tool called ghc-toolchain to configure toolchains and properties of targets into a /target description file/. The build system has been upgraded to read these target descriptions, but it does not yet read the targets created by ghc-toolchain. Instead, ./configure will also generate target description files (one for the host, one for the target) and hadrian will choose those by default.

For the purpose of beta-testing, we encourage users to invoke ./configure --enable-ghc-toolchain to configure the toolchain using ghc-toolchain, and report bugs!

(Ultimately, we'll move to using ghc-toolchain by default, so catching the bugs in this validation stage is much better than later on.)

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NB: During development in !9263 (merged) we came across multiple tasks and bugs regarding the configuration script. In preparation of merging it, we're opening tickets for the leftover tasks both regarding the new ghc-toolchain and the toolchain logic in configure. This is one of those tickets.