ghc-toolchain followup: Move the remaining toolchain settings to Toolchain

In !9263 (merged) we removed almost all settings-xxx from the hadrian configuration in favour of reading those settings from the relevant Target description file.

The glaring omissions, that weren't immediately needed for the patch, are:

• otool
• install-name-tool
• llc
• opt
• llvm-as

Out of these, touch is dropped in !9237 (closed), and I think distro-mingw can also be dropped or renamed to not mention settings. The other would be good additions as fields to GHC.Toolchain.Target.Target

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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.