change custom link section to support apple mach-o format

[spookyvision]

No description provided.

[hawkw]

It feels quite weird to me that we have to do this, since this code should never actually be built for a MacOS target: the decl_test! macro is only used in code that goes into the Mycelium kernel, which should always be built with target_os = "none".

So, perhaps we could simplify this to just:

#[cfg_attr(target_os = "none", link_section = "MyceliumTests")]

to avoid emitting the attribute at all when not building a Mycelium kernel image?

With that said, if the current code fixes the build on MacOS dev machines, I'm fine with it either way!

BTW, I believe this fixes issue #393, so let's add that to the commit message before merging!

[hawkw]

So, perhaps we could simplify this to just:

#[cfg_attr(target_os = "none", link_section = "MyceliumTests")]

to avoid emitting the attribute at all when not building a Mycelium kernel image?

I don't actually know whether this will work or not, FWIW. If the macOS linker is involved even when linking cross-compiled builds for x86_64-unknown-none, we might have to placate it like this anyway? Up to you if you want to investigate whether we can get away with just not having the attribute.