xctool 0.1.7

0.1.7 introduces a new directory layout where the xctool files are stored in
bin, lib, and libexec dirs -- https://github.com/facebook/xctool/commit/d6592a7

The old formula will probably break for anyone that tries to use --HEAD :-(

Closes #21199.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>