components: Fix support for 0-sized flags (#4560)

This commit goes through and updates support in the various argument
passing routines to support 0-sized flags. A bit of a degenerate case
but clarified in WebAssembly/component-model#76 as intentional.

crates/environ/src/fact/trampoline.rs

@@ -700,6 +700,7 @@ impl Compiler<'_, '_> {
         assert_eq!(src_ty.names, dst_ty.names);
         let cnt = src_ty.names.len();
         match FlagsSize::from_count(cnt) {
+            FlagsSize::Size0 => {}
             FlagsSize::Size1 => {
                 let mask = if cnt == 8 { 0xff } else { (1 << cnt) - 1 };
                 self.convert_u8_mask(src, dst, mask);