refactor: remove HADRIAN_SETTINGS environment variable

[angerman]

Summary

• Remove the fragile HADRIAN_SETTINGS environment variable mechanism
• Version and platform info is now generated directly by ./configure into template files

Changes

File	Change
configure.ac	Added platform detection (auto-detect from uname), added --with-target-arch, --with-target-os, --with-project-git-commit-id arguments
libraries/ghc-boot/GHC/Version.hs.in	New - Template for version info
libraries/ghc-boot/GHC/Platform/Host.hs.in	New - Template for platform types
Makefile	Removed HADRIAN_SETTINGS definition and all usages, updated CONFIGURED_FILES list
libraries/ghc-boot/Setup.hs	Simplified to just defaultMain (130 → 11 lines)
libraries/ghc-boot/ghc-boot.cabal.in	Changed build-type: Custom → Simple, removed custom-setup and autogen-modules

How It Works Now

1. Configure auto-detects platform using uname -m and uname -s, with override options:

   ./configure --with-target-arch=x86_64 --with-target-os=linux
   

2. Configure generates the modules directly from templates:

   • libraries/ghc-boot/GHC/Version.hs - version strings, git commit
   • libraries/ghc-boot/GHC/Platform/Host.hs - platform arch/OS types

3. No more environment variable passing between Makefile and Cabal

Benefits

• Simpler, more maintainable build system
• Single source of truth (configure.ac) for version/platform info
• Follows established pattern (like ghcversion.h.in)
• Easier to understand and debug
• Less fragile than environment variable mechanism

Test plan

• Run ./configure and verify generated files have correct content
• Build stage1: make CABAL=$PWD/_build/stage0/bin/cabal stage1
• Verify _build/stage1/bin/ghc --version shows correct version

[hsyl20]

If top-level configure does the substitution, how does it work when we build a cross-compiler (i.e. stage1 is non-cross and stage2 is cross)? The specified target should only apply to the stage2 compiler (stage1 is always non-cross).

[angerman]

If top-level configure does the substitution, how does it work when we build a cross-compiler (i.e. stage1 is non-cross and stage2 is cross)? The specified target should only apply to the stage2 compiler (stage1 is always non-cross).

Yea, I think we need to get rid of the target logic outright. This was just a first stab at nuking the HADRIAN_SETTINGS, as they pollute the Makefile.

Ultimately stage1 is always a native and stage2 is always a multi-target compiler (native + cross) compiler. In that case a single "target" is really not that meaningful. I think the solution is to just kill the target conditionals in a second step. For now this is just trying to do the same, just without

define HADRIAN_SETTINGS
[ ("hostPlatformArch",    "$(TARGET_ARCH)") \
, ("hostPlatformOS",      "$(TARGET_OS)") \
, ("cProjectGitCommitId", "$(GIT_COMMIT_ID)") \
, ("cProjectVersion",     "9.14") \
, ("cProjectVersionInt",  "914") \
, ("cProjectPatchLevel",  "0") \
, ("cProjectPatchLevel1", "0") \
, ("cProjectPatchLevel2", "0") \
]
endef