gdbstub: Clarify no more than @gdb_num_core_regs can be accessed

gdbstub: Clarify no more than @gdb_num_core_regs can be accessed

Both CPUClass::gdb_read_register() and CPUClass::gdb_write_register()
handlers are called from common gdbstub code, and won't be called with
register index over CPUClass::gdb_num_core_regs:

  int gdb_read_register(CPUState *cpu, GByteArray *buf, int reg)
  {
      CPUClass *cc = CPU_GET_CLASS(cpu);

      if (reg < cc->gdb_num_core_regs) {
          return cc->gdb_read_register(cpu, buf, reg);
      }
      ...
  }

  static int gdb_write_register(CPUState *cpu, uint8_t *mem_buf, int reg)
  {
      CPUClass *cc = CPU_GET_CLASS(cpu);

      if (reg < cc->gdb_num_core_regs) {
          return cc->gdb_write_register(cpu, mem_buf, reg);
      }
      ...
  }

Clarify that in CPUClass docstring, and remove unreachable code on
the microblaze and openrisc implementations.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250122093028.52416-3-philmd@linaro.org>