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).

cranelift/codegen/src/machinst/mod.rs

@@ -370,7 +370,7 @@ pub trait TextSectionBuilder {
     ///
     /// This function returns the offset at which the data was placed in the
     /// text section.
-    fn append(&mut self, labeled: bool, data: &[u8], align: u32) -> u64;
+    fn append(&mut self, labeled: bool, data: &[u8], align: Option<u32>) -> u64;
 
     /// Attempts to resolve a relocation for this function.
     ///