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