Ensure functions are aligned properly on AArch64 (#3908)

Previously (as in an hour ago) #3905 landed a new ability for fuzzing to
arbitrarily insert padding between functions. Running some fuzzers
locally though this instantly hit a lot of problems on AArch64 because
the arbitrary padding isn't aligned to 4 bytes like all other functions
are. To fix this issue appending functions now correctly aligns the
output as appropriate for the platform. The alignment argument for
appending was switched to `None` where `None` means "use the platform
default" and otherwise and explicit alignment can be specified for
inserting other data (like arbitrary padding or Windows unwind tables).

crates/cranelift/src/compiler.rs

@@ -264,9 +264,11 @@ impl wasmtime_environ::Compiler for Compiler {
             traps.push(range.clone(), &func.traps);
             func_starts.push(range.start);
             if self.linkopts.padding_between_functions > 0 {
-                builder
-                    .text
-                    .append(false, &vec![0; self.linkopts.padding_between_functions], 1);
+                builder.text.append(
+                    false,
+                    &vec![0; self.linkopts.padding_between_functions],
+                    Some(1),
+                );
             }
         }
 

crates/cranelift/src/obj.rs

@@ -184,12 +184,12 @@ impl<'a> ObjectBuilder<'a> {
     /// that the function resides within the text section.
     fn append_func(
         &mut self,
-        wat: bool,
+        labeled: bool,
         name: Vec<u8>,
         func: &'a CompiledFunction,
     ) -> (SymbolId, Range<u64>) {
         let body_len = func.body.len() as u64;
-        let off = self.text.append(wat, &func.body, 1);
+        let off = self.text.append(labeled, &func.body, None);
 
         let symbol_id = self.obj.add_symbol(Symbol {
             name,
@@ -215,7 +215,7 @@ impl<'a> ObjectBuilder<'a> {
                 let unwind_size = info.emit_size();
                 let mut unwind_info = vec![0; unwind_size];
                 info.emit(&mut unwind_info);
-                let unwind_off = self.text.append(false, &unwind_info, 4);
+                let unwind_off = self.text.append(false, &unwind_info, Some(4));
                 self.windows_unwind_info.push(RUNTIME_FUNCTION {
                     begin: u32::try_from(off).unwrap(),
                     end: u32::try_from(off + body_len).unwrap(),