Add option to generate BTF debug info for bpf programs

Add "btf" option that generates BTF debug info to support easier map
inspection. This is accomplished by passing the "-g" flag to clang
when compiling the BPF program.

The "-g" option also generates a number of DWARF debug sections which
are not necessary for loading BTF information, so strip these to avoid
increasing file size unnecessarily. bpfloader currently only supports
BTF info for maps, not programs, so we also strip the .BTF.ext section
containing program BTF info.

Bug: 203823368
Test: libbpf_load_test
Test: verify time_in_state.o includes .BTF section iff "btf: true" is
set
Test: verify time_in_state.o still loads if BTF is enabled
Change-Id: Ica25b253bace59d04130b0210350188399889bbe
Signed-off-by: Connor O'Brien <connoro@google.com>