debug.zig: prevent checking posix.system.msync on Wasi

There is an msync symbol when compiling against Wasi, but its creates
dependencies on the C library in that environment that seem to be
problematic for std.debug.  To be consistent with the old debug.zig, just
force have_msync false in the Wasi environment.

lib/std/debug.zig

@@ -839,7 +839,8 @@ pub const StackIterator = struct {
     }
 };
 
-const have_msync = @TypeOf(posix.system.msync) != void;
+// WASI has an msync, but its not good enough (See PR #20845)
+const have_msync = native_os != .wasi and @TypeOf(posix.system.msync) != void;
 
 pub fn writeCurrentStackTrace(
     out_stream: anytype,