Fixed style bugs in clock.c rev.1.164 and cpu.h rev.1.52-1.53 -- declare

Fixed style bugs in clock.c rev.1.164 and cpu.h rev.1.52-1.53 -- declare
tsc_present in the right places (together with other variables of the
same linkage), and don't use messy ifdefs just to avoid exporting it in
some cases.