gdb: new $_inferior_thread_count convenience variable

gdb: new $_inferior_thread_count convenience variable

Add a new convenience variable $_inferior_thread_count that contains
the number of live (non-exited) threads in the current inferior.  This
can be used in command scripts, or breakpoint conditions, etc to
adjust the behaviour for multi-threaded inferiors.

This value is only stable in all-stop mode.  In non-stop mode, where
new threads can be started, and existing threads exit, at any time,
this convenience variable can give a different value each time it is
evaluated.