Fix test failure on macOS with link_section

https://github.com/rust-lang/rust/pull/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 https://github.com/rust-lang/reference/issues/2245