Remove or explain target triple normalization

Currently during configure, GHC normalizes the triple passed e.g. with --target. It's not clear why this happens, and it has caused problems in the past (See #12840 (closed), #12487 (closed)), so we should probably either just remove the GHC triples altogether or add some explanations for why they're around.

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Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Build System
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changed weight to 5

added Tbug Trac import labels

The new Hadrian build system has been merged. Relabeling the tickets concerning the legacy make build system to prevent confusion.

Trac metadata

Trac field	Value
Component	Build System → Build System (make)

@Ericson2314, this is right down your ally.

As far as why we do this normalization: I don't know.

assigned to @Ericson2314

Hehe. Sure I'll take it. Once make is gone, I want to do a huge dead code purge of the configure stuff.

I can sort sort of see why this would happen, config.sub is very forgiving since it accepts both concrete machines and sets of machines roughly corresponding to GCC backends. But there are probably other better ways to achieve the same thing. (Perhaps the Platform parsing already achieves this a better way.)

added Pnormal label