Thread options & clone events (native Linux)

This commit teaches the native Linux target about the
GDB_THREAD_OPTION_CLONE thread option.  It's actually simpler to just
continue reporting all clone events unconditionally to the core.
There's never any harm in reporting a clone event when the option is
disabled.  All we need to do is to report support for the option,
otherwise GDB falls back to use target_thread_events().

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=19675
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27830


Reviewed-By: Andrew Burgess <aburgess@redhat.com>
Change-Id: If90316e2dcd0c61d0fefa0d463c046011698acf9