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/llvm/mod.rs

@@ -37,6 +37,12 @@ pub fn AddFunctionAttrStringValue(llfn: &'a Value, idx: AttributePlace, attr: &C
     }
 }
 
+pub fn AddFunctionAttrString(llfn: &'a Value, idx: AttributePlace, attr: &CStr) {
+    unsafe {
+        LLVMRustAddFunctionAttrStringValue(llfn, idx.as_uint(), attr.as_ptr(), std::ptr::null())
+    }
+}
+
 #[derive(Copy, Clone)]
 pub enum AttributePlace {
     ReturnValue,