<rdar://problem/13788579>

TestRegisters failing on buildbot on MacOSX. Some registers that were valid for linux/freebsd, were not valid on MacOSX. The tests now allow a register be skipped if it doesn't exist on the host.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@180961 91177308-0d34-0410-b5e6-96231b3b80d8