AAPCS64 says that it's up to the platform to specify whether x18 is
reserved, and a first step on that way is to add a flag controlling
it.
From: Andrew Turner <andrew@fubar.geek.nz>
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226664 91177308-0d34-0410-b5e6-96231b3b80d8