bpftool: Generated shadow variables for struct_ops maps.

Declares and defines a pointer of the shadow type for each struct_ops map.

The code generator will create an anonymous struct type as the shadow type
for each struct_ops map. The shadow type is translated from the original
struct type of the map. The user of the skeleton use pointers of them to
access the values of struct_ops maps.

However, shadow types only supports certain types of fields, including
scalar types and function pointers. Any fields of unsupported types are
translated into an array of characters to occupy the space of the original
field. Function pointers are translated into pointers of the struct
bpf_program. Additionally, padding fields are generated to occupy the space
between two consecutive fields.

The pointers of shadow types of struct_osp maps are initialized when
*__open_opts() in skeletons are called. For a map called FOO, the user can
access it through the pointer at skel->struct_ops.FOO.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20240229064523.2091270-4-thinker.li@gmail.com