support llvm-mingw for windows

[visualfc]

Support clang for windows

• clang target: x86_64-pc-windows-msvc. https://releases.llvm.org/
• clang target: x86_64-w64-windows-gnu. https://github.com/mstorsjo/llvm-mingw/releases
• add tags windows_msvc/windows_gnu

Changes

• c2go -test cleanEndLine for windows \r\n=>\n
• c2go use build.Import for support tags
• c2go check clang --version for parse ClangTarget to set tags windows_msvc/windows_gnu
• c2go proj use new pkg packages.NewImport for enable tags
• cl compileDeclStmt ast.FunctionDecl global or __mingw_** in func
• cl add blockCtx.needValist to need set "va_list" to ctypes.Valist if clang target windows-msvc
• clang/ast add kind DLLImportAttr UnusedAttr NoDebugAttr
• clang/types/parser fix https://github.com/goplus/c2go/issues/172 int (*)(void) attribute((cdecl)) for windows
• testdata add libc.go libc_unix.go libc_gnu_windows.go libc_msvc_windows.go
• testdata skip complex for _MSC_VER