Adjust global_asm doc test to work on all platforms.

The cfg isn't actually necessary, since `--emit=metadata` does not
actually do enough validation for it to fail on any platform. However,
I feel a little more comfortable leaving it in.

Unhide the feature flag, since I think that is important to display.

Author: ehuss

src/doc/unstable-book/src/library-features/global-asm.md

@@ -25,16 +25,17 @@ conventions of the assembler in your toolchain.
 A simple usage looks like this:
 
 ```rust,ignore (requires-external-file)
-# #![feature(global_asm)]
+#![feature(global_asm)]
 # // you also need relevant target_arch cfgs
 global_asm!(include_str!("something_neato.s"));
 ```
 
 And a more complicated usage looks like this:
 
 ```rust,no_run
-# #![feature(global_asm)]
-# #![cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+#![feature(global_asm)]
+# #[cfg(any(target_arch="x86", target_arch="x86_64"))]
+# mod x86 {
 
 pub mod sally {
     global_asm!(r#"
@@ -64,6 +65,7 @@ pub mod harry {
     #[no_mangle]
     pub unsafe extern "C" fn quux() {}
 }
+# }
 ```
 
 You may use `global_asm!` multiple times, anywhere in your crate, in