SDKROOT should only be unset/cleared for ARM builds. The first fix by Bob...

SDKROOT should only be unset/cleared for ARM builds. The first fix by Bob Wilson removed it for ARM for both configure and make, but only unset it for desktop configure, not for the make. My next fix unset/cleared it all the time. This fix only unset/clears it for ARM builds.

llvm-svn: 172593