tcg: add separate monitor command to dump opcode counters

Currently 'info jit' outputs half of the information to monitor and the
rest to qemu log. Dumping opcode counts to monitor as a part of 'info
jit' command doesn't sound useful. Add new monitor command 'info
opcount' that only dumps opcode counters.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>