Auto merge of #49101 - mark-i-m:stabilize_i128, r=nagisa

Stabilize 128-bit integers 🎉

cc #35118

EDIT: This should be merged only after the following have been merged:
- [x] rust-lang/compiler-builtins#236
- [x] rust-lang/book#1230

Author: bors

src/doc/unstable-book/src/language-features/i128-type.md

@@ -0,0 +1,18 @@
+# `repr128`
+
+The tracking issue for this feature is: [#35118]
+
+[#35118]: https://github.com/rust-lang/rust/issues/35118
+
+------------------------
+
+The `repr128` feature adds support for `#[repr(u128)]` on `enum`s.
+
+```rust
+#![feature(repr128)]
+
+#[repr(u128)]
+enum Foo {
+    Bar(u64),
+}
+```