Darwin builds: handle different possible form for SDKROOT.

Modifying build_llvm to handle SDKROOT being the name of an SDK rather than a
path. This will still work if SDKROOT is a path.

rdar://problem/15162322

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201560 91177308-0d34-0410-b5e6-96231b3b80d8