Add support for cmse_nonsecure_entry attribute

This patch adds support for the LLVM cmse_nonsecure_entry attribute.
This is a target-dependent attribute that only has sense for the
thumbv8m Rust targets.
You can find more information about this attribute here:
https://developer.arm.com/documentation/ecm0359818/latest/

Signed-off-by: Hugues de Valon <hugues.devalon@arm.com>

compiler/rustc_span/src/symbol.rs

@@ -337,6 +337,7 @@ symbols! {
         closure_to_fn_coercion,
         cmp,
         cmpxchg16b_target_feature,
+        cmse_nonsecure_entry,
         coerce_unsized,
         cold,
         column,