[lldb] [Process/FreeBSDRemote] Enable watchpoint support

Replace the inline x86 watchpoint handling code with the reusable
NativeRegisterContextWatchpoint_x86.  Implement watchpoint support
in NativeThreadFreeBSD and SIGTRAP handling for watchpoints.

Un-skip all concurrent_events tests as they pass with the new plugin.

Differential Revision: https://reviews.llvm.org/D90102