[compiler-rt] Fix running tests on macOS when XCode is not installed

[compiler-rt] Fix running tests on macOS when XCode is not installed

Summary:
If XCode is not installed, `xcodebuild -version -sdk macosx Path` will give
xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance
In this case the variable OSX_SYSROOT will be empty and
OSX_SYSROOT_FLAG is set to "-isysroot" (without a path).
This then causes the CompilerRTUnitTestCheckCxx target failed to for me
because "${COMPILER_RT_TEST_COMPILER} ${OSX_SYSROOT_FLAG} -E" expanded to
"clang -isysroot -E". This results in a warning "sysroot -E does not exist"
and the target fails to run because the C++ headers cannot be found.

Reviewers: beanz, kubamracek

Reviewed By: beanz

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

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D65323

llvm-svn: 367170