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/fuzzing/src/generators.rs

@@ -211,6 +211,8 @@ pub struct WasmtimeConfig {
     /// Configuration for the instance allocation strategy to use.
     pub strategy: InstanceAllocationStrategy,
     codegen: CodegenSettings,
+    padding_between_functions: Option<u16>,
+    generate_address_map: bool,
 }
 
 /// Configuration for linear memories in Wasmtime.
@@ -393,7 +395,8 @@ impl Config {
                 16 << 20,
                 self.wasmtime.memory_guaranteed_dense_image_size,
             ))
-            .allocation_strategy(self.wasmtime.strategy.to_wasmtime());
+            .allocation_strategy(self.wasmtime.strategy.to_wasmtime())
+            .generate_address_map(self.wasmtime.generate_address_map);
 
         self.wasmtime.codegen.configure(&mut cfg);
 
@@ -418,6 +421,16 @@ impl Config {
             }
         }
 
+        if let Some(pad) = self.wasmtime.padding_between_functions {
+            unsafe {
+                cfg.cranelift_flag_set(
+                    "wasmtime_linkopt_padding_between_functions",
+                    &pad.to_string(),
+                )
+                .unwrap();
+            }
+        }
+
         match &self.wasmtime.memory_config {
             MemoryConfig::Normal(memory_config) => {
                 cfg.static_memory_maximum_size(