aarch64: Fix push/pop_options with --with-cpu

aarch64: Fix push/pop_options with --with-cpu

If a toolchain is configured with --with-cpu=X and gcc is
then run with an explicit -march=Y option, we ignore the
X cpu setting and tune for generic Y code:

  if (!selected_cpu)
    {
      if (selected_arch)
        {
------>   selected_cpu = &all_cores[selected_arch->ident];
          aarch64_isa_flags = arch_isa;
          explicit_arch = selected_arch->arch;
        }
      else
        {
          /* Get default configure-time CPU.  */
          selected_cpu = aarch64_get_tune_cpu (aarch64_none);
          aarch64_isa_flags = TARGET_CPU_DEFAULT >> 6;
        }

      if (selected_tune)
        explicit_tune_core = selected_tune->ident;
    }
  …
  if (!selected_tune)
    selected_tune = selected_cpu;

But after a push/pop_options pair, we simply did:

  selected_tune = aarch64_get_tune_cpu (ptr->x_explicit_tune_core);

In the above scenario, ptr->x_explicit_tune_core is aarch64_none,
so we fall back on the default configure-time CPU.  This means
that before the push_options we tuned for generic Y but after
the pop_options we tuned for X.

This was picked up by an assertion failure in cl_optimization_compare.
The ICE itself is a GCC 11 regression, but the problem that it shows
up is much older.

gcc/
	* config/aarch64/aarch64.c (aarch64_option_restore): If the
	architecture was specified explicitly and the tuning wasn't,
	tune for the architecture rather than the configured default CPU.