Fuzz-code-coverage motivated improvements (#3905)

* fuzz: Fuzz padding between compiled functions

This commit hooks up the custom
`wasmtime_linkopt_padding_between_functions` configuration option to the
cranelift compiler into the fuzz configuration, enabling us to ensure
that randomly inserting a moderate amount of padding between functions
shouldn't tamper with any results.

* fuzz: Fuzz the `Config::generate_address_map` option

This commit adds fuzz configuration where `generate_address_map` is
either enabled or disabled, unlike how it's always enabled for fuzzing
today.

* Remove unnecessary handling of relocations

This commit removes a number of bits and pieces all related to handling
relocations in JIT code generated by Wasmtime. None of this is necessary
nowadays that the "old backend" has been removed (quite some time ago)
and relocations are no longer expected to be in the JIT code at all.
Additionally with the minimum x86_64 features required to run wasm code
it should be expected that no libcalls are required either for
Wasmtime-based JIT code.

crates/cranelift/src/compiler.rs

@@ -547,36 +547,22 @@ impl Compiler {
         isa: &dyn TargetIsa,
     ) -> Result<CompiledFunction, CompileError> {
         let mut code_buf = Vec::new();
-        let mut relocs = Vec::new();
         context
             .compile_and_emit(isa, &mut code_buf)
             .map_err(|error| CompileError::Codegen(pretty_error(&context.func, error)))?;
 
-        for &MachReloc {
-            offset,
-            srcloc: _,
-            kind,
-            ref name,
-            addend,
-        } in context
+        // Processing relocations isn't the hardest thing in the world here but
+        // no trampoline should currently generate a relocation, so assert that
+        // they're all empty and if this ever trips in the future then handling
+        // will need to be added here to ensure they make their way into the
+        // `CompiledFunction` below.
+        assert!(context
             .mach_compile_result
             .as_ref()
             .unwrap()
             .buffer
             .relocs()
-        {
-            let reloc_target = if let ir::ExternalName::LibCall(libcall) = *name {
-                RelocationTarget::LibCall(libcall)
-            } else {
-                panic!("unrecognized external name")
-            };
-            relocs.push(Relocation {
-                reloc: kind,
-                reloc_target,
-                offset,
-                addend,
-            });
-        }
+            .is_empty());
 
         let unwind_info = context
             .create_unwind_info(isa)
@@ -585,7 +571,7 @@ impl Compiler {
         Ok(CompiledFunction {
             body: code_buf,
             unwind_info,
-            relocations: relocs,
+            relocations: Vec::new(),
             stack_slots: Default::default(),
             value_labels_ranges: Default::default(),
             info: Default::default(),