bpf: Define bpf_jit_alloc_exec_limit for arm64 JIT

Expose the maximum amount of useable memory from the arm64 JIT.

Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211014142554.53120-3-lmb@cloudflare.com