Clear UNIVERSAL_SDK_PATH setting when building host tools. <rdar://12360497>

I also moved the SDKROOT setting into the make flags, since clearing it from
the environment isn't good enough to override a setting on the make command
line.  That hasn't been a problem but it could be, and it's good to be
consistent with the way UNIVERSAL_SDK_PATH is handled.

llvm-svn: 164565