Enable libffi support on Apple targets

[alexrp]

graf/build.zig

Lines 135 to 136 in b44ad9c

	// TODO: https://github.com/ziglang/zig/issues/20361
	else => !target.isDarwin(),

Requires:

• Bundled macOS headers cannot be found using #include in assembly files ziglang/zig#20361

[alexrp]

This can be done in Zig 0.14.0.