EXTRA_HC_OPTS workflow under hadrian.

This allows passing of arbitrary flags to ghc from the command line.

The main use case for me so far is to generate dumps for an arbitrary combination of dump flags.

Sometimes I wanted to look at the simplifier output for a certain file.

Sometimes I wanted to dump the asm code for all files to see how common certain patterns are. Sometimes the same for Cmm. Sometimes I want to dump two passes to be able to cross reference between them.

These are usually on-off things and the exact flags often differ. So imo custom build profiles are not a good solution for this.

Trac metadata

Trac field	Value
Version	8.6.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Build System (Hadrian)
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