Label WASI stat as kernel_stat

We're using WASI syscalls, it makes sense to put the kernel_ prefix.

lib/std/os/bits/wasi.zig

@@ -31,7 +31,7 @@ pub const timespec = struct {
     }
 };
 
-pub const libc_stat = struct {
+pub const kernel_stat = struct {
     dev: device_t,
     ino: inode_t,
     mode: mode_t,