[AArch64] Crash with low shared_ibt_table_bb_init value

While working on PR #4638, I found that ibl-stress.c with -DTEST_FAR_LINK_AARCH64 on AArch64 crashes with -shared_ibt_table_bb_init 16 and 18, but works with 20. Similar behaviour was observed on a proprietary app where -shared_ibt_table_bb_init had to be raised to avoid a crash. This was observed to happen with large apps only, which is why -DTEST_FAR_LINK_AARCH64 is required to reproduce the crash.