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_codegen_llvm/src/attributes.rs

@@ -294,6 +294,9 @@ pub fn from_fn_attrs(cx: &CodegenCx<'ll, 'tcx>, llfn: &'ll Value, instance: ty::
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::ALLOCATOR) {
         Attribute::NoAlias.apply_llfn(llvm::AttributePlace::ReturnValue, llfn);
     }
+    if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::CMSE_NONSECURE_ENTRY) {
+        llvm::AddFunctionAttrString(llfn, Function, const_cstr!("cmse_nonsecure_entry"));
+    }
     sanitize(cx, codegen_fn_attrs.no_sanitize, llfn);
 
     // Always annotate functions with the target-cpu they are compiled for.