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Remove dynamic-by-default (#16782)

Dynamic-by-default was a mechanism to automatically select the -dynamic way for some targets.

It was implemented in a convoluted way: it was defined as a flavour option, hence it couldn't be passed as a global settings (which are produced by configure before considering flavours), so a build system rule was used to pass -DDYNAMIC_BY_DEFAULT to the C compiler so that deriveConstants could infer it.

  • Make build system has it disabled for 8 years (951e28c0)
  • It has never been implemented in Hadrian
  • Last time someone tried to enable it 1 year ago it didn't work (!2436 (closed))
  • Having this as a global constant impedes making GHC multi-target (see !5427 (closed))

This commit fully removes support for dynamic-by-default. If someone wants to reimplement something like this, it would probably need to move the logic in the compiler.

(Doing this would probably need some refactoring of the way the compiler handles DynFlags: DynFlags are used to store and to pass enabled ways to many parts of the compiler. It can be set by command-line flags, GHC API, global settings. In multi-target GHC, we will use DynFlags to load the target platform and its constants: but at this point with the current DynFlags implementation we can't easily update the existing DynFlags with target-specific options such as dynamic-by-default without overriding ways previously set by the user.)

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