FROMLIST: rust: add flags for shadow call stack sanitizer

Add flags to support the shadow call stack sanitizer, both in the
dynamic and non-dynamic modes.

Right now, the compiler will emit the warning "unknown feature specified
for `-Ctarget-feature`: `reserve-x18`". However, the compiler still
passes it to the codegen backend, so the flag will work just fine. Once
rustc starts recognizing the flag (or provides another way to enable the
feature), it will stop emitting this warning. See [1] for the relevant
issue.

Currently, the compiler thinks that the aarch64-unknown-none target
doesn't support -Zsanitizer=shadow-call-stack, so the build will fail if
you enable shadow call stack in non-dynamic mode. However, I still think
it is reasonable to add the flag now, as it will at least fail the build
when using an invalid configuration, until the Rust compiler is fixed to
list -Zsanitizer=shadow-call-stack as supported for the target. See [2]
for the feature request to add this.

I have tested this change with Rust Binder on an Android device using
CONFIG_DYNAMIC_SCS. Without the -Ctarget-feature=+reserve-x18 flag, the
phone crashes immediately on boot, and with the flag, the phone appears
to work normally.

This contains a TODO to add the -Zuse-sync-unwind=n flag. The flag
defaults to n, so it isn't a problem today, but the flag is unstable, so
the default could change in a future compiler release.

Link: https://github.com/rust-lang/rust/issues/121970 [1]
Link: https://github.com/rust-lang/rust/issues/121972

 [2]
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

Bug: 328033850
Link: https://lore.kernel.org/rust-for-linux/20240305-shadow-call-stack-v2-1-c7b4a3f4d616@google.com/


Change-Id: Ia55287e1ed6da2d5d8d3d6414f2d9a0fc7d23e81
Signed-off-by: Alice Ryhl <aliceryhl@google.com>