BPF: change btf_type_tag BTF output format

BPF: change btf_type_tag BTF output format

For the declaration like below:
  int __tag1 * __tag1 __tag2 *g
Commit 41860e602aaa ("BPF: Support btf_type_tag attribute")
implemented the following encoding:
  VAR(g) -> __tag1 --> __tag2 -> pointer -> __tag1 -> pointer -> int

Some further experiments with linux btf_type_tag support, esp.
with generating attributes in vmlinux.h, and also some internal
discussion showed the following format is more desirable:
  VAR(g) -> pointer -> __tag2 -> __tag1 -> pointer -> __tag1 -> int

The format makes it similar to other modifier like 'const', e.g.,
  const int *g
which has encoding VAR(g) -> PTR -> CONST -> int

Differential Revision: https://reviews.llvm.org/D113496