rust: macros: fix make rusttest build on macOS

Do not emit `#[link_section = ".modinfo"]` on macOS (i.e. when building
userspace tests); .modinfo is not a legal section specifier in mach-o.

Before this change tests failed to compile:

  ---- ../rust/macros/lib.rs - module (line 66) stdout ----
  rustc-LLVM ERROR: Global variable '_ZN8rust_out13__module_init13__module_init27__MY_DEVICE_DRIVER_MODULE_017h141f80536770e0d4E' has an invalid section specifier '.modinfo': mach-o section specifier requires a segment and section separated by a comma.
  Couldn't compile the test.
  ---- ../rust/macros/lib.rs - module (line 33) stdout ----
  rustc-LLVM ERROR: Global variable '_ZN8rust_out13__module_init13__module_init20__MY_KERNEL_MODULE_017h5d79189564b41e07E' has an invalid section specifier '.modinfo': mach-o section specifier requires a segment and section separated by a comma.
  Couldn't compile the test.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Link: https://lore.kernel.org/r/20250210-macros-section-v2-1-3bb9ff44b969@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/macros/module.rs

@@ -56,7 +56,7 @@ impl<'a> ModInfoBuilder<'a> {
             "
                 {cfg}
                 #[doc(hidden)]
-                #[link_section = \".modinfo\"]
+                #[cfg_attr(not(target_os = \"macos\"), link_section = \".modinfo\")]
                 #[used]
                 pub static __{module}_{counter}: [u8; {length}] = *{string};
             ",