ci: windows: use builtin GHC clang toolchain to build dependencies

[elopez]

MSYS2 recently got its Clang updated from v18 to v19, and that caused build failures such as the following:

data-dword > [3 of 3] Compiling Data.DoubleWord
data-dword > ghc-9.6.6.exe: loadArchive: GNU-variant filename offset 31 invalid (range [0..964]) while reading filename from `D:\a_temp\msys64\clang64\lib\libmingw32.a'
data-dword >
data-dword > <no location info>: error:
data-dword > loadArchive "D:\a\_temp\msys64\clang64\lib\libmingw32.a": failed

This PR switches the Windows build to use the toolchain shipped together with GHC to avoid the incompatibility issues. It also adds a Windows-specific libff patch carried by hevm to fix precompile accuracy on Windows.