[libc++] Run macOS buildbot under xcrun (#167072)
This ensures that run-buildbot can find the appropriate build tools
without having to go through a bunch of hoops inside run-buildbot
itself. When we eventually run the bootstrapping build on macOS, it also
allows the bootstrapped Clang to find the SDK headers it requires.
This also allows simplifying run-buildbot a bunch since we should be
able to find build tools in the current $PATH.
