bpftool: Make skeleton C++ compatible with explicit casts

Fix C++ compilation errors in generated skeleton by adding explicit
pointer casts and use char * subtraction for offset calculation

error: invalid conversion from 'void*' to '<obj_name>*' [-fpermissive]
| skel = skel_alloc(sizeof(*skel));
| ~~~~~~~~~~^~~~~~~~~~~~~~~
| |
| void*

error: arithmetic on pointers to void
| skel->ctx.sz = (void *)&skel->links - (void *)skel;
| ~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~

error: assigning to 'struct <obj_name>__<ident> *' from incompatible type 'void *'
| skel-><ident> = skel_prep_map_data((void *)data, 4096,
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| sizeof(data) - 1);
| ~~~~~~~~~~~~~~~~~

error: assigning to 'struct <obj_name>__<ident> *' from incompatible type 'void *'
| skel-><ident> = skel_finalize_map_data(&skel->maps.<ident>.initial_value,
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| 4096, PROT_READ | PROT_WRITE, skel->maps.<ident>.map_fd);
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Minimum reproducer:

$ cat test.bpf.c
int val; // placed in .bss section

#include "vmlinux.h"
#include <bpf/bpf_helpers.h>

SEC("raw_tracepoint/sched_wakeup_new") int handle(void *ctx) { return 0; }

$ cat test.cpp
#include <cerrno>

extern "C" {
#include "test.bpf.skel.h"
}

$ bpftool btf dump file /sys/kernel/btf/vmlinux format c > vmlinux.h
$ clang -g -O2 -target bpf -c test.bpf.c -o test.bpf.o
$ bpftool gen skeleton test.bpf.o -L > test.bpf.skel.h
$ g++ -c test.cpp -I.

Co-developed-by: Menglong Dong <dongml2@chinatelecom.cn>
Signed-off-by: WanLi Niu <niuwl1@chinatelecom.cn>
Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260106023123.2928-1-kiraskyler@163.com