BPF: rename BTF_KIND_TAG to BTF_KIND_DECL_TAG

Per discussion in https://reviews.llvm.org/D111199,
the existing btf_tag attribute will be renamed to
btf_decl_tag. This patch updated BTF backend to
use btf_decl_tag attribute name and also
renamed BTF_KIND_TAG to BTF_KIND_DECL_TAG.

Differential Revision: https://reviews.llvm.org/D111592