Add Hadrian build artifacts to gitignore

Andrey wrote requesting the following,

For top-level .gitignore:

# -------------------------------------------------------------------------------------
# Hadrian's stuff. Note, **/stageN/ folders will be relocated to _build/ after
# resolving https://github.com/snowleopard/hadrian/issues/113
# -------------------------------------------------------------------------------------
_build/
hadrian/
**/stage?/

For .gitignore in affected submodules (array, deepseq, directory, filepath, hoopl, hpc, process, utils/hsc2hs):

# -------------------------------------------------------------------------------------
# Hadrian's stuff. Note, stageN/ folders will be relocated to top-level _build/
# after resolving https://github.com/snowleopard/hadrian/issues/113
# -------------------------------------------------------------------------------------
stage?/

Trac metadata

Trac field	Value
Version	8.0.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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