Prevents a seg fault in the burst_aarch64_sys test that was caused by reading an unspecified sigaction restorer in sig_has_restorer() in unix/signal.c. Does so by returning false early in sig_has_restorer() for AArch64 when the SA_RESTORER flag is not set.
By preventing the seg fault, it also prevents the nested signal handling and consequently the stack overflow in burst_aarch64_sys test when the -signal_stack_size is not specified.
Issue: #4425 (closed)