tools/bpftool: Strip away modifiers from global variables

Reliably remove all the type modifiers from read-only (.rodata) global
variable definitions, including cases of inner field const modifiers and
arrays of const values.

Also modify one of selftests to ensure that const volatile struct doesn't
prevent user-space from modifying .rodata variable.

Fixes: 985ead41 ("bpftool: Add skeleton codegen command")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200713232409.3062144-3-andriin@fb.com