rust: Kbuild: set frame-pointer llvm module flag for CONFIG_FRAME_POINTER

rust: Kbuild: set frame-pointer llvm module flag for CONFIG_FRAME_POINTER

Due to a rustc bug, the -Cforce-frame-pointers=y flag only emits the
frame-pointer annotation for functions, but not for the module. This
means that functions generated by the LLVM backend such as
'asan.module_ctor' do not receive the frame-pointer annotation.

This is likely to lead to broken backtraces and may also cause issues
with ftrace if these features are used with functions generated by the
LLVM backend.

Thus, use -Zllvm_module_flag to work around this rustc bug if using a
rustc without the fix.

[ The fix [1] has landed for Rust 1.98.0 (expected release on
  2026-08-20). - Miguel ]

Cc: stable@vger.kernel.org # 6.12.y and later (flag not available in pinned Rust in older LTSs).
Fixes: 2f7ab1267dc9 ("Kbuild: add Rust support")
Link: https://github.com/rust-lang/rust/pull/156980
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260616-frame-ptr-fix-v1-1-dc6b29a631d9@google.com
[ - Adjusted Cc: stable@ as discussed.

  - Added comment with link to the PR, similar to what we did in commit
    ac35b5580ace ("rust: arm64: set uwtable llvm module flag for
    CONFIG_UNWIND_TABLES").

    - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>