[compiler-rt] Use xcrun instead of xcodebuild to find the SDK directory

Summary:
xcodebuild does not work unless XCode is installed whereas xcrun also work
when only the Command Line Tools are installed. Unlike the check for the
version (D69610), this did not cause an erro for me since the fallback to
/usr/include for the OSX sysroot worked.

Reviewers: yln, delcypher

Reviewed By: yln

Subscribers: dberris, mgorny, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D69659