FreeBSD hardware watchpoint implementation

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