`alloc`: make `vec!` unavailable under `no_global_oom_handling`

The `vec!` macro has 3 rules, but two are not usable under
`no_global_oom_handling` builds of the standard library
(even with a zero size):

```rust
let _ = vec![42];    // Error: requires `exchange_malloc` lang_item.
let _ = vec![42; 0]; // Error: cannot find function `from_elem`.
```

Thus those two rules should not be available to begin with.

The remaining one, with an empty matcher, is just a shorthand for
`new()` and may not make as much sense to have alone, since the
idea behind `vec!` is to enable `Vec`s to be defined with the same
syntax as array expressions. Furthermore, the documentation can be
confusing since it shows the other rules.

Thus perhaps it is better and simpler to disable `vec!` entirely
under `no_global_oom_handling` environments, and let users call
`new()` instead:

```rust
let _: Vec<i32> = vec![];
let _: Vec<i32> = Vec::new();
```

Notwithstanding this, a `try_vec!` macro would be useful, such as
the one introduced in https://github.com/rust-lang/rust/pull/95051.

If the shorthand for `new()` is deemed worth keeping on its own,
then it may be interesting to have a separate `vec!` macro with
a single rule and different, simpler documentation.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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