Remove #[fixed_stack_segment] and #[rust_stack]

These two attributes are no longer useful now that Rust has decided to leave
segmented stacks behind. It is assumed that the rust task's stack is always
large enough to make an FFI call (due to the stack being very large).

There's always the case of stack overflow, however, to consider. This does not
change the behavior of stack overflow in Rust. This is still normally triggered
by the __morestack function and aborts the whole process.

C stack overflow will continue to corrupt the stack, however (as it did before
this commit as well). The future improvement of a guard page at the end of every
rust stack is still unimplemented and is intended to be the mechanism through
which we attempt to detect C stack overflow.

Closes #8822
Closes #10155

src/librustdoc/html/render.rs

@@ -717,7 +717,6 @@ impl Context {
                       port: &SharedPort<Work>,
                       chan: &SharedChan<Work>,
                       prog_chan: &SharedChan<Progress>) {
-                #[fixed_stack_segment]; // we hit markdown FFI *a lot*
                 local_data::set(cache_key, cache);
 
                 loop {