Use custom entry name in gcc

This is a continuation of 9f0a862 for
gcc.

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

compiler/rustc_codegen_gcc/src/context.rs

@@ -425,8 +425,9 @@ impl<'gcc, 'tcx> MiscMethods<'tcx> for CodegenCx<'gcc, 'tcx> {
     }
 
     fn declare_c_main(&self, fn_type: Self::Type) -> Option<Self::Function> {
-        if self.get_declared_value("main").is_none() {
-            Some(self.declare_cfn("main", fn_type))
+        let entry_name = self.sess().target.entry_name.as_ref();
+        if self.get_declared_value(entry_name).is_none() {
+            Some(self.declare_entry_fn(entry_name, fn_type, ()))
         }
         else {
             // If the symbol already exists, it is an error: for example, the user wrote

compiler/rustc_codegen_gcc/src/declare.rs

@@ -65,13 +65,13 @@ impl<'gcc, 'tcx> CodegenCx<'gcc, 'tcx> {
         global
     }
 
-    pub fn declare_cfn(&self, name: &str, _fn_type: Type<'gcc>) -> RValue<'gcc> {
+    pub fn declare_entry_fn(&self, name: &str, _fn_type: Type<'gcc>, callconv: () /*llvm::CCallConv*/) -> RValue<'gcc> {
         // TODO(antoyo): use the fn_type parameter.
         let const_string = self.context.new_type::<u8>().make_pointer().make_pointer();
         let return_type = self.type_i32();
         let variadic = false;
         self.linkage.set(FunctionType::Exported);
-        let func = declare_raw_fn(self, name, () /*llvm::CCallConv*/, return_type, &[self.type_i32(), const_string], variadic);
+        let func = declare_raw_fn(self, name, callconv, return_type, &[self.type_i32(), const_string], variadic);
         // NOTE: it is needed to set the current_func here as well, because get_fn() is not called
         // for the main function.
         *self.current_func.borrow_mut() = Some(func);