selftests/bpf: Add a test for btf_type_tag "percpu"

Add test for percpu btf_type_tag. Similar to the "user" tag, we test
the following cases:

 1. __percpu struct field.
 2. __percpu as function parameter.
 3. per_cpu_ptr() accepts dynamically allocated __percpu memory.

Because the test for "user" and the test for "percpu" are very similar,
a little bit of refactoring has been done in btf_tag.c. Basically, both
tests share the same function for loading vmlinux and module btf.

Example output from log:

 > ./test_progs -v -t btf_tag

 libbpf: prog 'test_percpu1': BPF program load failed: Permission denied
 libbpf: prog 'test_percpu1': -- BEGIN PROG LOAD LOG --
 ...
 ; g = arg->a;
 1: (61) r1 = *(u32 *)(r1 +0)
 R1 is ptr_bpf_testmod_btf_type_tag_1 access percpu memory: off=0
 ...
 test_btf_type_tag_mod_percpu:PASS:btf_type_tag_percpu 0 nsec
 #26/6 btf_tag/btf_type_tag_percpu_mod1:OK

 libbpf: prog 'test_percpu2': BPF program load failed: Permission denied
 libbpf: prog 'test_percpu2': -- BEGIN PROG LOAD LOG --
 ...
 ; g = arg->p->a;
 2: (61) r1 = *(u32 *)(r1 +0)
 R1 is ptr_bpf_testmod_btf_type_tag_1 access percpu memory: off=0
 ...
 test_btf_type_tag_mod_percpu:PASS:btf_type_tag_percpu 0 nsec
 #26/7 btf_tag/btf_type_tag_percpu_mod2:OK

 libbpf: prog 'test_percpu_load': BPF program load failed: Permission denied
 libbpf: prog 'test_percpu_load': -- BEGIN PROG LOAD LOG --
 ...
 ; g = (__u64)cgrp->rstat_cpu->updated_children;
 2: (79) r1 = *(u64 *)(r1 +48)
 R1 is ptr_cgroup_rstat_cpu access percpu memory: off=48
 ...
 test_btf_type_tag_vmlinux_percpu:PASS:btf_type_tag_percpu_load 0 nsec
 #26/8 btf_tag/btf_type_tag_percpu_vmlinux_load:OK

 load_btfs:PASS:could not load vmlinux BTF 0 nsec
 test_btf_type_tag_vmlinux_percpu:PASS:btf_type_tag_percpu 0 nsec
 test_btf_type_tag_vmlinux_percpu:PASS:btf_type_tag_percpu_helper 0 nsec
 #26/9 btf_tag/btf_type_tag_percpu_vmlinux_helper:OK

Signed-off-by: Hao Luo <haoluo@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220304191657.981240-5-haoluo@google.com