Add core::arch::breakpoint and test

Approved in [ACP 491](https://github.com/rust-lang/libs-team/issues/491).

Remove the `unsafe` on `core::intrinsics::breakpoint()`, since it's a
safe intrinsic to call and has no prerequisites.

(Thanks to @zachs18 for figuring out the `bootstrap`/`not(bootstrap)`
logic.)

compiler/rustc_error_codes/src/error_codes/E0622.md

@@ -7,10 +7,11 @@ Erroneous code example:
 #![allow(internal_features)]
 
 extern "rust-intrinsic" {
-    pub static breakpoint: fn(); // error: intrinsic must be a function
+    pub static atomic_singlethreadfence_seqcst: fn();
+    // error: intrinsic must be a function
 }
 
-fn main() { unsafe { breakpoint(); } }
+fn main() { unsafe { atomic_singlethreadfence_seqcst(); } }
 ```
 
 An intrinsic is a function available for use in a given programming language
@@ -22,8 +23,8 @@ error, just declare a function. Example:
 #![allow(internal_features)]
 
 extern "rust-intrinsic" {
-    pub fn breakpoint(); // ok!
+    pub fn atomic_singlethreadfence_seqcst(); // ok!
 }
 
-fn main() { unsafe { breakpoint(); } }
+fn main() { unsafe { atomic_singlethreadfence_seqcst(); } }
 ```