The interpreter instruction count was enabled based on RUBY_DEBUG as
opposed to YJIT_STATS. In builds with YJIT_STATS=1 but RUBY_DEBUG=0,
the count was not available.

Move YJIT_STATS in yjit.h where declarations are expoed to code outside
of YJIT. Also reduce the changes made to the interpreter for calling
into YJIT's instruction counting function.