bpftool: Strip const/volatile/restrict modifiers from .bss and .data vars

Similarly to .rodata, strip any const/volatile/restrict modifiers when
generating BPF skeleton. They are not helpful and actually just get in the way.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210507054119.270888-2-andrii@kernel.org