x86/time: switch platform timer hooks to altcall

x86/time: switch platform timer hooks to altcall

Except in the "clocksource=tsc" case we can replace the indirect calls
involved in accessing the platform timers by direct ones, as they get
established once and never changed. To also cover the "tsc" case, invoke
what read_tsc() resolves to directly. In turn read_tsc() then becomes
unreachable and hence can move to .init.*.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>