selftests/bpf: Fix verifier_arena_globals1 failure with 64K page

selftests/bpf: Fix verifier_arena_globals1 failure with 64K page

With 64K page on arm64, verifier_arena_globals1 failed like below:
  ...
  libbpf: map 'arena': failed to create: -E2BIG
  ...
  #509/1   verifier_arena_globals1/check_reserve1:FAIL
  ...

For 64K page, if the number of arena pages is (1UL << 20), the total
memory will exceed 4G and this will cause map creation failure.
Adjusting ARENA_PAGES based on the actual page size fixed the problem.

Cc: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260113061033.3798549-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>