Use ptr::cast instead of as casts in several places. (#3507)

`ptr::cast` has the advantage of being unable to silently cast
`*const T` to `*mut T`. This turned up several places that were
performing such casts, which this PR also fixes.

crates/jit/src/unwind/systemv.rs

@@ -21,8 +21,8 @@ impl UnwindRegistration {
     /// describe an in-memory representation of a `.eh_frame` section. This is
     /// typically arranged for by the `wasmtime-obj` crate.
     pub unsafe fn new(
-        _base_address: *mut u8,
-        unwind_info: *mut u8,
+        _base_address: *const u8,
+        unwind_info: *const u8,
         unwind_len: usize,
     ) -> Result<UnwindRegistration> {
         debug_assert_eq!(

crates/jit/src/unwind/winx64.rs

@@ -11,8 +11,8 @@ pub struct UnwindRegistration {
 
 impl UnwindRegistration {
     pub unsafe fn new(
-        base_address: *mut u8,
-        unwind_info: *mut u8,
+        base_address: *const u8,
+        unwind_info: *const u8,
         unwind_len: usize,
     ) -> Result<UnwindRegistration> {
         assert!(unwind_info as usize % 4 == 0);