Replace MaybeUninit::uninit_array() with array repeat expression.

This is possible now that inline const blocks are stable; the idea was
even mentioned as an alternative when `uninit_array()` was added:
<https://github.com/rust-lang/rust/pull/65580#issuecomment-544200681>

> if it’s stabilized soon enough maybe it’s not worth having a
> standard library method that will be replaceable with
> `let buffer = [MaybeUninit::<T>::uninit(); $N];`

Const array repetition and inline const blocks are now stable (in the
next release), so that circumstance has come to pass, and we no longer
have reason to want `uninit_array()` other than convenience. Therefore,
let’s evaluate the inconvenience by not using `uninit_array()` in
the standard library, before potentially deleting it entirely.

library/std/src/process/tests.rs

@@ -137,7 +137,7 @@ fn child_stdout_read_buf() {
     let child = cmd.spawn().unwrap();
 
     let mut stdout = child.stdout.unwrap();
-    let mut buf: [MaybeUninit<u8>; 128] = MaybeUninit::uninit_array();
+    let mut buf: [MaybeUninit<u8>; 128] = [MaybeUninit::uninit(); 128];
     let mut buf = BorrowedBuf::from(buf.as_mut_slice());
     stdout.read_buf(buf.unfilled()).unwrap();