selftests/bpf: utility function to get program disassembly after jit

This commit adds a utility function to get disassembled text for jited
representation of a BPF program designated by file descriptor.
Function prototype looks as follows:

    int get_jited_program_text(int fd, char *text, size_t text_sz)

Where 'fd' is a file descriptor for the program, 'text' and 'text_sz'
refer to a destination buffer for disassembled text.
Output format looks as follows:

    18:	77 06                               	ja	L0
    1a:	50                                  	pushq	%rax
    1b:	48 89 e0                            	movq	%rsp, %rax
    1e:	eb 01                               	jmp	L1
    20:	50                                  L0:	pushq	%rax
    21:	50                                  L1:	pushq	%rax
     ^  ^^^^^^^^                             ^  ^^^^^^^^^^^^^^^^^^
     |  binary insn                          |  textual insn
     |  representation                       |  representation
     |                                       |
    instruction offset              inferred local label name

The code and makefile changes are inspired by jit_disasm.c from bpftool.
Use llvm libraries to disassemble BPF program instead of libbfd to avoid
issues with disassembly output stability pointed out in [1].

Selftests makefile uses Makefile.feature to detect if LLVM libraries
are available. If that is not the case selftests build proceeds but
the function returns -EOPNOTSUPP at runtime.

[1] commit eb9d1acf ("bpftool: Add LLVM as default library for disassembling JIT-ed programs")

Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240820102357.3372779-6-eddyz87@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>