kexec: use mem_text_write_kernel_word to set reboot_code_buffer args

* In order to avoid protection faults (writes to read-only
  kernel memory) when CONFIG_STRICT_MEMORY_RWX is enabled.

Signed-off-by: Vojtech Bocek <vbocek@gmail.com>
Signed-off-by: Francisco Franco <franciscofranco.1990@gmail.com>