Thread options & clone events (native Linux)

Thread options & clone events (native Linux)

This commit teaches the native Linux target about the GDB_TO_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 report support for the option, otherwise GDB falls back to use
target_thread_events().

Change-Id: If271f20320d864f074d8ac0d531cc1a323da847f
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=19675
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27830