Improve generating Custom entry function

This commit is aimed at making compiler generated entry functions
(Basically just C `main` right now) more generic so other targets can do
similar things for custom entry. This was initially implemented as part
of https://github.com/rust-lang/rust/pull/100316.

Currently, this moves the entry function name and Call convention to the
target spec.

Signed-off-by: Ayush Singh <ayushsingh1325@gmail.com>

compiler/rustc_codegen_llvm/src/declare.rs

@@ -90,6 +90,28 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
         declare_raw_fn(self, name, llvm::CCallConv, unnamed, visibility, fn_type)
     }
 
+    /// Declare an entry Function
+    ///
+    /// The ABI of this function can change depending on the target (although for now the same as
+    /// `declare_cfn`)
+    ///
+    /// If there’s a value with the same name already declared, the function will
+    /// update the declaration and return existing Value instead.
+    pub fn declare_entry_fn(
+        &self,
+        name: &str,
+        callconv: llvm::CallConv,
+        unnamed: llvm::UnnamedAddr,
+        fn_type: &'ll Type,
+    ) -> &'ll Value {
+        let visibility = if self.tcx.sess.target.default_hidden_visibility {
+            llvm::Visibility::Hidden
+        } else {
+            llvm::Visibility::Default
+        };
+        declare_raw_fn(self, name, callconv, unnamed, visibility, fn_type)
+    }
+
     /// Declare a Rust function.
     ///
     /// If there’s a value with the same name already declared, the function will