Use bundled llc/opt on Windows

Motivation

Since Windows bindist for GHC 9.4+ comes with llc.exe/opt.exe, GHC should use them by default.

lib/settings should contain something like:

,("LLVM llc command", "$tooldir/mingw/bin/llc.exe")
,("LLVM opt command", "$tooldir/mingw/bin/opt.exe")

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