Mark core::alloc::Layout::from_size_align_unchecked const

[richard-w]

Makes it possible (pending stabilization of #57563 (const_fn)) to rewrite code like

const BUFFER_SIZE: usize = 0x2000;
const BUFFER_ALIGN: usize = 0x1000;

fn foo() {
  let layout = std::alloc::Layout::from_size_align(BUFFER_SIZE, BUFFER_ALIGN)
    .unwrap();
  let buffer = std::alloc::alloc(layout);
}

to

const BUFFER_LAYOUT: std::alloc::Layout = unsafe {
  std::alloc::Layout::from_size_align_unchecked(0x2000, 0x1000)
};

fn foo() {
  let buffer = std::alloc::alloc(BUFFER_LAYOUT);
}

which (although unsafe is used) looks somewhat cleaner and is easier to read.