Revert the Darwin half of r184805: "Remove the sysroot (or isysroot)
restriction from the GCDAProfile.c"

Bill added a much more effective way of coping with the use of errno and
other system interactions for Darwin for now by disabling it on that
platform. Building against the SDK stuff is more fundamental to the
darwin build process.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@184929 91177308-0d34-0410-b5e6-96231b3b80d8