Remove change to set SDKROOT when building compiler-rt on Darwin.

This reverts the runtime library portion of r194168. As of r221621,
the libclang_rt libraries for Darwin build with explicit SDK options
so there is no need to set SDKROOT here.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221625 91177308-0d34-0410-b5e6-96231b3b80d8