unstable book: in a sanitizer example, check the code

this uses some # directives to make sure the code works on x86_64, and does not produce errors on other platforms

Author: folkertdev

src/doc/unstable-book/src/compiler-flags/sanitizer.md

@@ -244,10 +244,11 @@ See the [Clang ControlFlowIntegrity documentation][clang-cfi] for more details.
 
 ## Example 1: Redirecting control flow using an indirect branch/call to an invalid destination
 
-```rust,ignore (making doc tests pass cross-platform is hard)
+```rust
 #![feature(naked_functions)]
 
-use std::arch::asm;
+# #[cfg(target_arch = "x86_64")] mod m {
+use std::arch::naked_asm;
 use std::mem;
 
 fn add_one(x: i32) -> i32 {
@@ -258,22 +259,19 @@ fn add_one(x: i32) -> i32 {
 pub extern "C" fn add_two(x: i32) {
     // x + 2 preceded by a landing pad/nop block
     unsafe {
-        asm!(
-            "
-             nop
-             nop
-             nop
-             nop
-             nop
-             nop
-             nop
-             nop
-             nop
-             lea eax, [rdi+2]
-             ret
-        ",
-            options(noreturn)
-        );
+        naked_asm!(
+            "nop",
+            "nop",
+            "nop",
+            "nop",
+            "nop",
+            "nop",
+            "nop",
+            "nop",
+            "nop",
+            "lea eax, [rdi+2]",
+            "ret",
+        )
     }
 }
 
@@ -297,6 +295,8 @@ fn main() {
 
     println!("The next answer is: {}", next_answer);
 }
+# }
+# #[cfg(not(target_arch = "x86_64"))] fn main() {}
 ```
 Fig. 1. Redirecting control flow using an indirect branch/call to an invalid
 destination (i.e., within the body of the function).