make mingw .def.in file parsing use aro's preprocessor instead of clang

[andrewrk]

This code converts mingw's .def.in files to .def files:

zig/src/mingw.zig

Lines 291 to 293 in 1c85b0a

	// We need to invoke `zig clang` to use the preprocessor.
	if (!build_options.have_llvm) return error.ZigCompilerNotBuiltWithLLVMExtensions;
	const self_exe_path = comp.self_exe_path orelse return error.PreprocessorDisabled;

zig/src/mingw.zig

Lines 348 to 375 in 1c85b0a

	const args = [_][]const u8{
	self_exe_path,
	"clang",
	"-x",
	"c",
	def_file_path,
	"-Wp,-w",
	"-undef",
	"-P",
	"-I",
	try comp.zig_lib_directory.join(arena, &[_][]const u8{ "libc", "mingw", "def-include" }),
	target_def_arg,
	"-E",
	"-o",
	def_final_path,
	};
	if (comp.verbose_cc) {
	Compilation.dump_argv(&args);
	}
	if (std.process.can_spawn) {
	var child = std.ChildProcess.init(&args, arena);
	child.stdin_behavior = .Ignore;
	child.stdout_behavior = .Pipe;
	child.stderr_behavior = .Pipe;
	try child.spawn();

Instead of using clang's preprocessor, it should use Aro's.

Related:

• #17752

cc @Vexu @ehaas