arm64: rust: Enable PAC support for Rust.

Enable the PAC ret and BTI options in the Rust build flags to match
the options that are used when building C.

Signed-off-by: Jamie Cunliffe <Jamie.Cunliffe@arm.com>
Link: https://lore.kernel.org/r/20230606145606.1153715-3-Jamie.Cunliffe@arm.com

Author: JamieCunliffe

arch/arm64/Makefile

@@ -67,7 +67,9 @@ endif
 
 ifeq ($(CONFIG_ARM64_BTI_KERNEL),y)
   KBUILD_CFLAGS += -mbranch-protection=pac-ret+bti
+  KBUILD_RUSTFLAGS += -Z branch-protection=bti,pac-ret
 else ifeq ($(CONFIG_ARM64_PTR_AUTH_KERNEL),y)
+  KBUILD_RUSTFLAGS += -Z branch-protection=pac-ret
   ifeq ($(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET),y)
     KBUILD_CFLAGS += -mbranch-protection=pac-ret
   else