Update doc/BuildingNe10.txt for Linux
Update doc/BuildingNe10.txt for Linux

Now, users need to set NE10_LINUX_TARGET_ARCH for native compiling Ne10
on Linux.

Change-Id: I56ce3187f39b07ea5408051d606be7d8dd62f9af

arnopaehler/Ne10,h6ah4i/Ne10,h6ah4i/Ne10,h6ah4i/Ne10,arnopaehler/Ne10,arnopaehler/Ne10,h6ah4i/Ne10,arnopaehler/Ne10,arnopaehler/Ne10,h6ah4i/Ne10