coff: fix incorrect default image_base values and re-enable shared library tests on Windows

[kcbanner]

Closes #16965
Closes #16959
Closes #16960
Closes #18427

As part of investigating a CI failure on aarch64-windows for #21758, I noticed that all the shared library tests were being skipped on either windows or aarch64-windows. I did some testing in an aarch64-windows VM, and noticed that the error happening here was a "Bad Image" error coming from the dll.

The way I figured this out was by removing each of the optional arguments from the linker line to see if I could get a different result. As it turned out, removing the -BASE:268435456 argument for the shared library caused the bug to go away. It turns out this value is incorrect for 64-bit targets, see: https://learn.microsoft.com/en-us/cpp/build/reference/base-base-address?view=msvc-170

Passing the correct value of 0x180000000 resolved the issue, which led me to the diff in this PR.

The regression that introduced this bug (as #18427 hints at) was this change in #18160:

-        if (self.base.options.image_base_override) |image_base| {
-            try argv.append(try std.fmt.allocPrint(arena, "-BASE:{d}", .{image_base}));
+        try argv.append(try std.fmt.allocPrint(arena, "-BASE:{d}", .{self.image_base}));

Instead of sending the optional override, self.image_base (which then contained the incorrect value for a 64-bit images) was now always being set on the linker line.

This didn't seem to cause an issue on x86_64, but it seems aarch64 Windows is more strict about this, and an image created with the wrong BASE would result in this "Bad Image" error there.

This PR fixes the image_base value as well as re-enables the disabled tests.

[andrewrk]

Nice sleuthing, thank you.