Don't use #[no_mangle] (clashes with `#[export_name]`)

This closes #270.

With this, the symbol name is correct in the intermediate LLVM bitcode
object file (`.rcgu.o`) and in the final BPF program.

bpf/aya-bpf-macros/src/expand.rs

@@ -97,7 +97,6 @@ impl Map {
         let name = &self.name;
         let item = &self.item;
         Ok(quote! {
-            #[no_mangle]
             #[link_section = #section_name]
             #[export_name = #name]
             #item