rust: module: place cleanup_module() in .exit.text section

Place cleanup_module() in .exit.text section. Currently,
cleanup_module() is likely placed in the .text section. It's
inconsistent with the layout of C modules, where cleanup_module() is
placed in .exit.text.

[ Boqun asked for an example of how the section changed to be
  put in the log. Tomonori provided the following examples:

    C module:

      $ objdump -t ~/build/x86/drivers/block/loop.o|grep clean
      0000000000000000 l     O .exit.data    0000000000000008 __UNIQUE_ID___addressable_cleanup_module412
      0000000000000000 g     F .exit.text    000000000000009c cleanup_module

    Rust module without this patch:

      $ objdump -t ~/build/x86/samples/rust/rust_minimal.o|grep clean
      00000000000002b0 g     F .text         00000000000000c6 cleanup_module
      0000000000000000 g     O .exit.data    0000000000000008 _R...___UNIQUE_ID___addressable_cleanup_module

    Rust module with this patch:

      $ objdump -t ~/build/x86/samples/rust/rust_minimal.o|grep clean
      0000000000000000 g     F .exit.text    00000000000000c6 cleanup_module
      0000000000000000 g     O .exit.data    0000000000000008 _R...___UNIQUE_ID___addressable_cleanup_module

  - Miguel ]

Signed-off-by: FUJITA Tomonori <[email protected]>
Acked-by: Jarkko Sakkinen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: fujita

rust/macros/module.rs

@@ -278,6 +278,7 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
                     #[cfg(MODULE)]
                     #[doc(hidden)]
                     #[no_mangle]
+                    #[link_section = \".exit.text\"]
                     pub extern \"C\" fn cleanup_module() {{
                         // SAFETY:
                         // - This function is inaccessible to the outside due to the double