Fix test failure on macOS with link_section

rust-lang/rust#155065 recently added validation
for link_section on macOS. I'm working around it here by just limiting
this test to linux.

Alternatives:
- Show different syntaxes for different targets (but I would prefer to
  keep the example as plain as possible).
- Mark it as `ignore` (want to avoid this if at all possible).

Fixes rust-lang#2245

Author: ehuss

src/abi.md

@@ -98,9 +98,11 @@ The `link_section` attribute uses the [MetaNameValueStr] syntax to specify the s
 
 <!-- no_run: don't link. The format of the section name is platform-specific. -->
 ```rust,no_run
+# #[cfg(target_os = "linux")] {
 #[unsafe(no_mangle)]
 #[unsafe(link_section = ".example_section")]
 pub static VAR1: u32 = 1;
+# }
 ```
 
 r[abi.link_section.unsafe]