By adding a feature test for btf__raw_data() and providing a fallback if
it isn't present in older versions of libbpf.

Committer testing:

  $ rpm -q libbpf-devel
  libbpf-devel-0.4.0-2.fc35.x86_64
  $ make -C tools/perf LIBBPF_DYNAMIC=1 O=/tmp/build/perf install-bin
  $ cat /tmp/build/perf/feature/test-libbpf-btf__raw_data.make.output
  test-libbpf-btf__raw_data.c: In function ‘main’:
  test-libbpf-btf__raw_data.c:6:9: error: implicit declaration of function ‘btf__raw_data’; did you mean ‘btf__get_raw_data’? [-Werror=implicit-function-declaration]
      6 |         btf__raw_data(NULL /* btf_ro */, NULL /* size */);
        |         ^~~~~~~~~~~~~
        |         btf__get_raw_data
  cc1: all warnings being treated as errors
  $ objdump -dS /tmp/build/perf/perf | grep '<btf__raw_data>:' -A20
  00000000005b3050 <btf__raw_data>:
  {
    5b3050:	55                   	push   %rbp
    5b3051:	48 89 e5             	mov    %rsp,%rbp
    5b3054:	48 83 ec 10          	sub    $0x10,%rsp
    5b3058:	64 48 8b 04 25 28 00 	mov    %fs:0x28,%rax
    5b305f:	00 00
    5b3061:	48 89 45 f8          	mov    %rax,-0x8(%rbp)
    5b3065:	31 c0                	xor    %eax,%eax
	  return btf__get_raw_data(btf_ro, size);
    5b3067:	48 8b 45 f8          	mov    -0x8(%rbp),%rax
    5b306b:	64 48 2b 04 25 28 00 	sub    %fs:0x28,%rax
    5b3072:	00 00
    5b3074:	75 06                	jne    5b307c <btf__raw_data+0x2c>
  }
    5b3076:	c9                   	leave
	  return btf__get_raw_data(btf_ro, size);
    5b3077:	e9 14 99 e5 ff       	jmp    40c990 <btf__get_raw_data@plt>
    5b307c:	e8 af a7 e5 ff       	call   40d830 <__stack_chk_fail@plt>
    5b3081:	66 66 2e 0f 1f 84 00 	data16 cs nopw 0x0(%rax,%rax,1)
    5b3088:	00 00 00 00
    $
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Link: http://lore.kernel.org/linux-perf-users/YozLKby7ITEtchC9@kravaSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>