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_hir_analysis/src/check/intrinsic.rs

@@ -87,6 +87,7 @@ pub fn intrinsic_operation_unsafety(tcx: TyCtxt<'_>, intrinsic_id: LocalDefId) -
         | sym::assert_inhabited
         | sym::assert_zero_valid
         | sym::assert_mem_uninitialized_valid
+        | sym::breakpoint
         | sym::size_of
         | sym::min_align_of
         | sym::needs_drop