Fix compiler-rt build on macOS without XCode
Summary:
Starting with
8a5bfbe6db2824642bf9a1d27a24c5b6132b244f (D68292) this file
unconditionally uses xcodebuild to get the SDK version. On my system this
always fails with
`xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance`
Reviewers: delcypher, yln
Reviewed By: delcypher, yln
Subscribers: dberris, mgorny, #sanitizers, llvm-commits
Tags: #sanitizers, #llvm
Differential Revision: https://reviews.llvm.org/D69610