It is too risky to rely on the compiler to not insert any operations using the
stack.
Having a stack-setting call in Rust using the cortex-a crate as the first action
in a Rust-only _start() function does not work if you're subsequently using the
stack, because the compiler often inserts the operations to make room on the
stack to prepare a function call BEFORE the call to set the stack, which crashes
the boot process.
Hence, keep on using a small piece of assembly boot code throughout.
