[build][rust] Enable Shadow Call Stack for Rust on AArch64 and RISC-V

After rust-lang/rust#124676 was rolled, we can
enable SCS for both AArch64 and RISC-V, since it won't conflict with
other sanitizers in the compiler driver anymore.

Fixed: 42069386
Bug: 360955800
Change-Id: Ib30747204882317bb6b243c8a8ce760f09520a83
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/791504
Size-Review: Przemek Pietrzkiewicz <ppi@google.com>
Fuchsia-Auto-Submit: Paul Kirth <paulkirth@google.com>
Reviewed-by: Roland McGrath <mcgrathr@google.com>
Commit-Queue: Paul Kirth <paulkirth@google.com>

build/config/arm64/BUILD.gn

@@ -16,7 +16,10 @@ config("arm64") {
   # rustc --target=aarch64-unknown-fuchsia --print target-features
   # `armv8a` and `simd` features are assumed by the target triple
   # `aarch64-unknown-fuchsia`.
-  rustflags = [ "-Ctarget-feature=+crc,+aes,+sha2" ]
+  rustflags = [
+    "-Ctarget-feature=+crc,+aes,+sha2",
+    "-Zsanitizer=shadow-call-stack",
+  ]
 }
 
 config("cortex-a53") {

build/config/riscv64/BUILD.gn

@@ -25,6 +25,9 @@ config("riscv64") {
     cflags = [ "-msmall-data-limit=0" ]
   }
   configs = [ ":abi" ]
+
+  # TODO(https://fxbug.dev/360955800): remove the SCS option once the rustc driver can set it by default for Fuchsia targets
+  rustflags = [ "-Zsanitizer=shadow-call-stack" ]
 }
 
 riscv64_abi_config("no-vector") {