Implement x86_64 debug register read/write in support of hardware
watchpoints. Hoist LinuxThread::TraceNotify code back into
POSIXThread::TraceNotify()
Patch by John Wolfe.
We still need to rework this later to avoid the #ifdef FreeBSD.
llvm-reviews.chandlerc.com/D2572
llvm.org/pr16706
llvm-svn: 201706