Add detection of OS X relocatable SDK to compiler-rt as a lit.util function

Clang's lit cfg already detects the currently selected SDK via
"xcrun --show-sdk-path". The same thing should be done for compiler-rt tests,
to make them work on recent OS X versions. Instead of duplicating the detection
code, this patch extracts the detection function into a lit.util method.

Patch by Kuba Brecka (kuba.brecka@gmail.com),
reviewed at http://reviews.llvm.org/D4072


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@210534 91177308-0d34-0410-b5e6-96231b3b80d8