Remove morestack dependency.

[ababo]

As we know the concept of segmented stack has been already abandoned. But for freestanding build the Rust compiler still complains about missing libmorestack.a:

rustc --crate-type rlib,staticlib --target x86_64-unknown-linux-gnu -C opt-level=3 -C no-stack-check -Z no-landing-pads --cfg arch_x86_64 --sysroot /dev/null -L /path/to/lib.rs -o /path/to/libcore.rlib

error: could not find native static library morestack, perhaps an -L flag is missing?

I suppose this dependency should be removed.

[alexcrichton]

I believe this is covered by #16012, so closing in favor of that.