Move atomic-from-mut-not-available.rs to tests/ui/stdlib-unit-tests/

This test exercises the combined effect of the
`cfg(target_has_atomic_equal_alignment = "...")` implementation in the
compiler as well as the usage of said
`cfg(target_has_atomic_equal_alignment)` in `core`.

tests/ui/atomic-from-mut-not-available.stderr

@@ -1,17 +0,0 @@
-error[E0599]: no function or associated item named `from_mut` found for struct `AtomicU64` in the current scope
-  --> $DIR/atomic-from-mut-not-available.rs:5:36
-   |
-LL |     core::sync::atomic::AtomicU64::from_mut(&mut 0u64);
-   |                                    ^^^^^^^^ function or associated item not found in `AtomicU64`
-   |
-note: if you're trying to build a new `AtomicU64`, consider using `AtomicU64::new` which returns `AtomicU64`
-  --> $SRC_DIR/core/src/sync/atomic.rs:LL:COL
-   = note: this error originates in the macro `atomic_int` (in Nightly builds, run with -Z macro-backtrace for more info)
-help: there is an associated function `from` with a similar name
-   |
-LL |     core::sync::atomic::AtomicU64::from(&mut 0u64);
-   |                                    ~~~~
-
-error: aborting due to 1 previous error
-
-For more information about this error, try `rustc --explain E0599`.