cpu: Directly use get_memory_mapping() fallback handlers in place

cpu: Directly use get_memory_mapping() fallback handlers in place

No code uses CPUClass::get_memory_mapping() outside of hw/core/cpu.c:

  $ git grep -F -- '->get_memory_mapping'
  hw/core/cpu.c:87:    cc->get_memory_mapping(cpu, list, errp);
  hw/core/cpu.c:439:    k->get_memory_mapping = cpu_common_get_memory_mapping;
  target/i386/cpu.c:7422:    cc->get_memory_mapping = x86_cpu_get_memory_mapping;

Check the handler presence in place and remove the common fallback code.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210517105140.1062037-11-f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>