Use empty() instead of NONE with rsix flags types.

`empty()` is provided by all `bitflags` types, so it's more idiomatic
than having `NONE` values.

crates/runtime/src/instance/allocator/pooling/unix.rs

@@ -14,7 +14,7 @@ fn decommit(addr: *mut u8, len: usize, protect: bool) -> Result<()> {
             addr as _,
             len,
             if protect {
-                rsix::io::ProtFlags::NONE
+                rsix::io::ProtFlags::empty()
             } else {
                 rsix::io::ProtFlags::READ | rsix::io::ProtFlags::WRITE
             },

crates/runtime/src/mmap.rs

@@ -190,7 +190,7 @@ impl Mmap {
                 rsix::io::mmap_anonymous(
                     ptr::null_mut(),
                     mapping_size,
-                    rsix::io::ProtFlags::NONE,
+                    rsix::io::ProtFlags::empty(),
                     rsix::io::MapFlags::PRIVATE,
                 )
                 .context(format!("mmap failed to allocate {:#x} bytes", mapping_size))?

crates/runtime/src/traphandlers/unix.rs

@@ -297,7 +297,7 @@ pub fn lazy_per_thread_init() -> Result<(), Box<Trap>> {
         let ptr = rsix::io::mmap_anonymous(
             null_mut(),
             alloc_size,
-            rsix::io::ProtFlags::NONE,
+            rsix::io::ProtFlags::empty(),
             rsix::io::MapFlags::PRIVATE,
         )
         .map_err(|_| Box::new(Trap::oom()))?;