Simplifications for xran builds (#273)

- make xran_DOWNLOAD=ON default - skips an extra argument in cmake
  command
- Automatically detect aarch64 and select correct target for external
  xran build
Reviewed-by: Jaroslava Fiedlerova <jaroslava.fiedlerova@openairinterface.org>
Reviewed-by: Teodora Vladić <teodora.vladic@openairinterface.org>