xen/timer: rename "timer.c" to "xen_timer.c"

[ehem_freebsd_m5p.com]

Summary

The better to avoid conflicting with other files given how "timer.c"
isn't a very unique name. ARM and RISC-V both have timer.c files.

[ehem_freebsd_m5p.com]

I can dump this on favored git repository sites if desired (https://gitlab.com/ehem/freebsd-src.git or https://github.com/ehem/freebsd-src.git branch would match the differential number).

It would be handy if @cperciva or @imp could accept and commit, since I already seem to be overloading @royger with patches. Pretty simple filename collision. Right now there aren't plans to get FreeBSD/Xen on RISC-V or ARM32, but steps have been taken to make those as easy as possible. ARM32 seems unlikely, but RISC-V looks quite likely in the future.

[royger]

I was expecting this would happen sooner or later, TBH I was waiting for a real clash to happen before doing the renaming. I assume you now need to use parts of this file on Arm?

[royger]

FWIW xen_time.c might be slightly better, because it contains the code for the event timer, but also the time counter and the system clock.

You will also need to update the folder name in that case

[imp]

I'm agnostic on royger's suggestion.
I note there's a number of other similar files as well, like debug.c..