Mitigate #[align] name resolution ambiguity regression with a rename

From `#[align]` -> `#[rustc_align]`. Attributes starting with `rustc`
are always perma-unstable and feature-gated by `feature(rustc_attrs)`.

See regression RUST-143834.

For the underlying problem where even introducing new feature-gated
unstable built-in attributes can break user code such as

```rs
macro_rules! align {
    () => {
        /* .. */
    };
}

pub(crate) use align; // `use` here becomes ambiguous
```

refer to RUST-134963.

Since the `#[align]` attribute is still feature-gated by
`feature(fn_align)`, we can rename it as a mitigation. Note that
`#[rustc_align]` will obviously mean that current unstable user code
using `feature(fn_aling)` will need additionally `feature(rustc_attrs)`,
but this is a short-term mitigation to buy time, and is expected to be
changed to a better name with less collision potential.

See
<https://rust-lang.zulipchat.com/#narrow/channel/238009-t-compiler.2Fmeetings/topic/.5Bweekly.5D.202025-07-17/near/529290371>
where mitigation options were considered.

Author: jieyouxu

tests/pass/fn_align.rs

@@ -1,15 +1,19 @@
 //@compile-flags: -Zmin-function-alignment=8
+
+// FIXME(rust-lang/rust#82232, rust-lang/rust#143834): temporarily renamed to mitigate `#[align]`
+// nameres ambiguity
+#![feature(rustc_attrs)]
 #![feature(fn_align)]
 
 // When a function uses `align(N)`, the function address should be a multiple of `N`.
 
-#[align(256)]
+#[rustc_align(256)]
 fn foo() {}
 
-#[align(16)]
+#[rustc_align(16)]
 fn bar() {}
 
-#[align(4)]
+#[rustc_align(4)]
 fn baz() {}
 
 fn main() {