i386/lapic.c: sync with amd64/lapic.c

i386/lapic.c: sync with amd64/lapic.c

In anticipation of merging the new clock interrupt code after release,
let's synchronize the lapic timer parts of i386's lapic.c with those
of amd64's lapic.c.  The two files will need similar changes to switch
to the new code, so the more alike they are the better.

Tested by mlarkin@ on an ESXi VM.  Tested by me on my amd64 laptop
running in x86 compatibility mode.

ok mlarkin@