Don't hardcode path to codesign_allocate.

Building OS X debugserver assumes you have an Xcode installation at /Application/Xcode.app. Let's instead detect where Xcode is using xcrun.

See http://reviews.llvm.org/D4152





git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@211074 91177308-0d34-0410-b5e6-96231b3b80d8