Fix compiler-rt build on macOS without XCode

Summary:
Starting with 8a5bfbe6 (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