make the compile-time logic in my_timer_cycles() also #define
MY_TIMER_ROUTINE_CYCLES to indicate which implementation it is using.
Then, make my_timer_init() use MY_TIMER_ROUTINE_CYCLES.

This leaves us with just one set of compile-time #if's which determine
how we read time in #cycles.

Reviewer (and commit message author): Sergei Petrunia <sergey@mariadb.com>