[BPF][DebugInfo] Use .BPF.ext for line info when DWARF is not available

"BTF" is a debug information format used by LLVM's BPF backend.
The format is much smaller in scope than DWARF, the following info is
available:
- full set of C types used in the binary file;
- types for global values;
- line number / line source code information .

BTF information is embedded in ELF as .BTF and .BTF.ext sections.
Detailed format description could be found as a part of Linux Source
tree, e.g. here: [1].

This commit modifies `llvm-objdump` utility to use line number
information provided by BTF if DWARF information is not available.
E.g., the goal is to make the following to print source code lines,
interleaved with disassembly:

    $ clang --target=bpf -g test.c -o test.o
    $ llvm-strip --strip-debug test.o
    $ llvm-objdump -Sd test.o

    test.o:	file format elf64-bpf

    Disassembly of section .text:

    <foo>:
    ; void foo(void) {
    	r1 = 0x1
    ;   consume(1);
    	call -0x1
    	r1 = 0x2
    ;   consu...