Shadow call stack should be supported on `aarch64-unknown-none`

[Darksonn]

Passing the -Zsanitizer=shadow-call-stack flag to rustc when compiling for the aarch64-unknown-none target results in the following error:

error: shadow-call-stack sanitizer is not supported for this target

However, as long as the x18 register is reserved during codegen (see #121970), there shouldn't be any problem with enabling the shadow call stack sanitizer on aarch64-unknown-none. It is already supported on several other aarch64 targets (see #121966).

The Linux Kernel uses the aarch64-unknown-none target when compiling Rust code for aarch64. This flag is needed so that we can enable CONFIG_SHADOW_CALL_STACK when using Rust in the kernel.

For more information on this sanitizer, please see the relevant chapter in the unstable book.

Related to this thread on LKML.