Turn cIntegerLibraryType into a dynflag

Currently, at configure time we tell GHC whether integers are built using integer-gmp or integer-simple, and this gets baked in via the global constant cIntegerLibraryType.

This is all fine for normal use of GHC (the program), but users of ghc (the library) might want to have more leeway here.

A good solution seems to be turn this into a dynflag setting, so that users of the GHC API can adjust that as fit. From a cursory glance at the code, this does not seem to be a big problem.

Trac metadata

Trac field	Value
Version	8.1
Type	Task
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	GHC API
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