eliminate the "default" code model

[andrewrk]

zig/lib/std/builtin.zig

Lines 116 to 129 in 69195d0

	/// The code model puts constraints on the location of symbols and the size of code and data.
	/// The selection of a code model is a trade off on speed and restrictions that needs to be selected on a per application basis to meet its requirements.
	/// A slightly more detailed explanation can be found in (for example) the [System V Application Binary Interface (x86_64)](https://github.com/hjl-tools/x86-psABI/wiki/x86-64-psABI-1.0.pdf) 3.5.1.
	///
	/// This data structure is used by the Zig language code generation and
	/// therefore must be kept in sync with the compiler implementation.
	pub const CodeModel = enum {
	default,
	tiny,
	small,
	kernel,
	medium,
	large,
	};

according to https://raw.githubusercontent.com/wiki/hjl-tools/x86-psABI/x86-64-psABI-1.0.pdf page 37, "default" is not an actual code model. This is out-of-bound data. The enum should not contain it. Instead, the type ?std.builtin.CodeModel should be used to represent the possibility of default.