Split the current NetBSD watchpoint implementation for x86 into Utility,
and revamp it to improve readability. This code is meant to be used
as a common class for all x86 watchpoint implementation, particularly
these on FreeBSD and Linux.
The code uses global watchpoint enable bits, as required by the NetBSD
kernel. If it ever becomes necessary for any platform to use local
enable bits instead, this can be trivially absracted out.
The code also postpones clearing DR6 until a new different watchpoint
is being set in place of the old one. This is necessary since LLDB
repeatedly reenables watchpoints on all threads, by clearing
and restoring them. When DR6 is cleared as a part of that, then pending
events on other threads can no longer be associated with watchpoints