kexec: Enable and fix kexec syscall support.
Use mem_text_write_kernel_word when assigning reboot_code_buffer parameters
to avoid protection faults (writes to read-only kernel memory) when
CONFIG_STRICT_MEMORY_RWX is enabled.

Orgin Code from https://github.com/mkasick/android_kernel_samsung_d2spr/commit/e0d4785c1f905a1510548830b0fab58c3a3cf880

Signed-off-by: cholokei <leesl0416@naver.com>