clock: Simplify subr_ticks and rename

• We can use builtin constants for the size of int and long to simplify definitions.
• The file should have a .S prefix since we want to run it through the preprocessor, though apparently this happens anyway with .s...
• Move ticks and ticksl from .data to .bss.

Reported by: jrtc27
Reviewed by: jrtc27, kib, emaste
Fixes: 6b82130e6c9a ("clock: Add a long ticks variable, ticksl")
Differential Revision: https://reviews.freebsd.org/D48420

(cherry picked from commit b2b974f7ef4cddff251d0de69d8da13232b25e4d)