bpf: Generate const static pointers for kernel helpers

The generated bpf_helper_defs.h file currently contains definitions
like this for the kernel helpers, which are static objects:

  static void *(*bpf_map_lookup_elem)(void *map, const void *key) = (void *) 1;

These work well in both clang and GCC because both compilers do
constant propagation with -O1 and higher optimization, resulting in
`call 1' BPF instructions being generated, which are calls to kernel
helpers.

However, there is a discrepancy on how the -Wunused-variable
warning (activated by -Wall) is handled in these compilers:

- clang will not emit -Wunused-variable warnings for static variables
  defined in C header files, be them constant or not constant.

- GCC will not emit -Wunused-variable warnings for _constant_ static
  variables defined in header files, but it will emit warnings for
  non-constant static variables defined in header files.

There is no reason for these bpf_helpers_def.h pointers to not be
declared constant, and it is actually desirable to do so, since their
values are not to be changed.  So this patch modifies bpf_doc.py to
generate prototypes like:

  static void *(* const bpf_map_lookup_elem)(void *map, const void *key) = (void *) 1;

This allows GCC to not error while compiling BPF programs with `-Wall
-Werror', while still being able to detect and error on legitimate
unused variables in the program themselves.

This change doesn't impact the desired constant propagation in neither
Clang nor GCC with -O1 and higher.  On the contrary, being declared as
constant may increase the odds they get constant folded when
used/referred to in certain circumstances.

Tested in bpf-next master.
No regressions.

Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20240127185031.29854-1-jose.marchesi@oracle.com