allow #[rustc_align_static(N)] on statics

We need a different attribute than `rustc_align` because unstable attributes are
tied to their feature (we can't have two unstable features use the same
unstable attribute). Otherwise this uses all of the same infrastructure
as `#[rustc_align]`.

Author: folkertdev

tests/pass/static_align.rs

@@ -0,0 +1,14 @@
+#![feature(static_align)]
+
+// When a static uses `align(N)`, its address should be a multiple of `N`.
+
+#[rustc_align_static(256)]
+static FOO: u64 = 0;
+
+#[rustc_align_static(512)]
+static BAR: u64 = 0;
+
+fn main() {
+    assert!(core::ptr::from_ref(&FOO).addr().is_multiple_of(256));
+    assert!(core::ptr::from_ref(&BAR).addr().is_multiple_of(512));
+}