Revert "New generated main returns void"

This reverts commit 267800a.

Author: bkchr

src/test/codegen/abi-main-signature-16bit-c-int.rs

@@ -29,4 +29,4 @@
 fn main() {
 }
 
-// CHECK: define void @main(i16, i8**)
+// CHECK: define i16 @main(i16, i8**)

src/test/codegen/abi-main-signature-32bit-c-int.rs

@@ -17,4 +17,4 @@
 fn main() {
 }
 
-// CHECK: define void @main(i32, i8**)
+// CHECK: define i32 @main(i32, i8**)