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.

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>

arch/x86/Makefile

@@ -81,6 +81,7 @@ ifeq ($(CONFIG_X86_KERNEL_IBT),y)
 #   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104816
 #
 KBUILD_CFLAGS += $(call cc-option,-fcf-protection=branch -fno-jump-tables)
+KBUILD_RUSTFLAGS += -Zcf-protection=branch -Zno-jump-tables
 else
 KBUILD_CFLAGS += $(call cc-option,-fcf-protection=none)
 endif