README: add caution of toolchain usages for 64bit OS
README: add caution of toolchain usages for 64bit OS

Our recommanded toolchain is fully working on 32bits OS.
64bits OS needs to install additional ubuntu package.
This commit describes above.

JeonginKim/TizenRT,junmin-kim/TizenRT,an4967/TizenRT,davidfather/TizenRT,chanijjani/TizenRT,sunghan-chang/TizenRT,jsdosa/TizenRT,junmin-kim/TizenRT,pillip8282/TizenRT,Samsung/TizenRT,jeongarmy/TizenRT,jsdosa/TizenRT,chanijjani/TizenRT,jeongchanKim/TizenRT,JeonginKim/TizenRT,an4967/TizenRT,jeongchanKim/TizenRT,btheosam/TizenRT,btheosam/TizenRT,sunghan-chang/TizenRT,pillip8282/TizenRT,jeongarmy/TizenRT,sunghan-chang/TizenRT,pillip8282/TizenRT,chanijjani/TizenRT,chanijjani/TizenRT,Samsung/TizenRT,davidfather/TizenRT,davidfather/TizenRT,an4967/TizenRT,chanijjani/TizenRT,davidfather/TizenRT,pillip8282/TizenRT,junmin-kim/TizenRT,jeongarmy/TizenRT,lokeshbv/TizenRT,JeonginKim/TizenRT,sunghan-chang/TizenRT,sunghan-chang/TizenRT,HONGCHAEHEE/TizenRT,Samsung/TizenRT,jsdosa/TizenRT,junmin-kim/TizenRT,chanijjani/TizenRT,lokeshbv/TizenRT,btheosam/TizenRT,jsdosa/TizenRT,an4967/TizenRT,jsdosa/TizenRT,HONGCHAEHEE/TizenRT,jsdosa/TizenRT,jeongarmy/TizenRT,junmin-kim/TizenRT,JeonginKim/TizenRT,jeongchanKim/TizenRT,sunghan-chang/TizenRT,Samsung/TizenRT,jeongarmy/TizenRT,lokeshbv/TizenRT,davidfather/TizenRT,Samsung/TizenRT,jeongchanKim/TizenRT,jeongarmy/TizenRT,pillip8282/TizenRT,JeonginKim/TizenRT,an4967/TizenRT,HONGCHAEHEE/TizenRT,JeonginKim/TizenRT,pillip8282/TizenRT,HONGCHAEHEE/TizenRT,jeongchanKim/TizenRT,jsdosa/TizenRT,Samsung/TizenRT,pillip8282/TizenRT,btheosam/TizenRT,HONGCHAEHEE/TizenRT,sunghan-chang/TizenRT,Samsung/TizenRT,davidfather/TizenRT,an4967/TizenRT,HONGCHAEHEE/TizenRT,btheosam/TizenRT,jeongarmy/TizenRT,lokeshbv/TizenRT,jeongchanKim/TizenRT,junmin-kim/TizenRT,lokeshbv/TizenRT,jeongchanKim/TizenRT,btheosam/TizenRT,junmin-kim/TizenRT,an4967/TizenRT,lokeshbv/TizenRT,davidfather/TizenRT,chanijjani/TizenRT