self-hosted ELF linker does not support weak externs

[mlugg]

Zig Version

0.15.0-dev.375+8f8f37fb0

Steps to Reproduce and Observed Behavior

pub fn main() void {
    // `var` to work around #23623
    var weak: ?*const fn () callconv(.c) void = undefined;
    weak = @extern(*const fn () callconv(.c) void, .{ .name = "does_not_exist", .linkage = .weak });

    if (weak) |f| {
        f();
    }
}

$ zig run repro.zig -fno-llvm
error: undefined symbol: does_not_exist
    note: referenced by repro.o:repro.main

Expected Behavior

A successful compilation and execution with no output, as happens when using LLVM:

$ zig run repro.zig -fllvm
$

If you build-obj and then nm the object file, you can see that the ELF linker just isn't marking the symbol as weak, whereas the LLVM backend does:

$ zig build-obj repro.zig -fllvm
$ nm repro.o | grep does_not_exist
                 w does_not_exist
$ zig build-obj repro.zig -fno-llvm
$ nm repro.o | grep does_not_exist
                 U does_not_exist

[Rexicon226]

Same issue with the MachO linker?

$ zig build-exe repro.zig
error: undefined symbol: _does_not_exist
    note: referenced by repro.o:_repro.main
    note: referenced by repro.o:_repro.main