UPSTREAM: x86: Enable IBT in Rust if enabled in C

These flags are not made conditional on compiler support because at the
moment exactly one version of rustc supported, and that one supports
these flags.

Building without these additional flags will manifest as objtool
printing a large number of errors about missing ENDBR and if CFI is
enabled (not currently possible) will result in incorrectly structured
function prefixes.

Change-Id: I07e70db23eaa7fb86f8ba7694000a6fea4332447
Signed-off-by: Matthew Maurer <mmaurer@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Link: https://lore.kernel.org/r/20231009224347.2076221-1-mmaurer@google.com


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
(cherry picked from commit 6a5c032c)