lib/std/start.zig ignores entry option

There is std.Build.Step.Compile.Entry

pub const Entry = union(enum) {
    /// Let the compiler decide whether to make an entry point and what to name
    /// it.
    default,
    /// The executable will have no entry point.
    disabled,
    /// The executable will have an entry point with the default symbol name.
    enabled,
    /// The executable will have an entry point with the specified symbol name.
    symbol_name: []const u8,
};

and the CLI flags -fentry and -fno-entry for specifying the name of the entry symbol of the executable or omitting an entry function altogether.
In lib/std/start.zig it assumes the name of the entry function:

const start_sym_name = if (native_arch.isMIPS()) "__start" else "_start";

The issue:

$ cat x.zig
pub export fn _start() void {}
$ zig build-exe x.zig # works

Now change _start to start and tell the compiler about it:

$ cat x.zig
pub export fn start() void {}
$ zig build-exe x.zig -fentry=start
zig-linux-x86_64/lib/std/start.zig:607:46: error: root struct of file 'x' has no member named 'main'

It should compile successfully.

Perhaps to fix this the Entry struct above should be moved into lib/std/builtin.zig (as std.builtin.Entry) and the entry option should be exposed through @import("builtin").
lib/std/start.zig then looks at @import("builtin").entry instead of assuming anything.