kbuild: rust: add AutoFDO support

kbuild: rust: add AutoFDO support

This patch enables AutoFDO build support for Rust code within the Linux
kernel. This allows Rust code to be profiled and optimized based on the
profile.

The RUSTFLAGS variable was suffixed with *_AUTOFDO_CLANG to match the
naming of the config option, which is called CONFIG_AUTOFDO_CLANG.

This implementation has been verified in Android, first by inspecting
the object files and confirming that they look correct. After that,
it was verified as below:

1. Running the binderAddInts benchmark [1] with Rust Binder built as
   rust_binder.ko module, using a Pixel 9 Pro.
2. Collecting a profile on a Pixel 10 Pro XL using the app-launch
   benchmark, which starts different apps many times, on a device with
   Rust Binder as a built-in kernel module. (C Binder was not present on
   the device.)
3. Using the collected profile, run the binderAddInts benchmark again
   with Rust Binder built both as a rust_binder.ko module, and as a
   built-in kernel module.
4. In both cases, Rust Binder without AutoFDO was approximately 13%
   slower than the AutoFDO optimized version. Built-in vs .ko did not
   make a measurable performance difference.

All of the above was verified in conjunction with my helpers inlining
series [2], which confirmed that this worked correctly for helpers too
once [3] was fixed in the helpers inlining series.

Link: https://android.googlesource.com/platform/system/extras/+/920f089/tests/binder/benchmarks/binderAddInts.cpp
Link: https://lore.kernel.org/r/20260203-inline-helpers-v2-0-beb8547a03c9@google.com
Link: https://lore.kernel.org/r/aasPsbMEsX6iGUl8@google.com
Reviewed-by: Rong Xu <xur@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Nicolas Schier <nsc@kernel.org>
Acked-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260331-autofdo-v2-1-eb5c5964820d@google.com
[ Reworded for typos. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>