Change custom section names to string slice

We will never need to create these names dynamically.
e.g. "reloc.CODE", "reloc.DATA", and "linking" will all
come from specific places in our backend code.
It's a different story for function names etc.

compiler/gen_wasm/src/lib.rs

@@ -109,7 +109,7 @@ pub fn build_module_help<'a>(
 
     // Code relocations
     let code_reloc_section = RelocationSection {
-        name: "reloc.CODE".to_string(),
+        name: "reloc.CODE",
         entries: &backend.code_relocations,
     };