usr.bin/top: fix displaying load average for loads of at least 100

[pkubaj]

Summary

After top registers load average of at least 100 which then gets reduced to
below 100, there are left stray digits.
Supporting load over 100 requires increasing the width only to 6, but since
we support over 1000 CPU's now, let's increase it to 7.

Test Plan

Run top while running with load over 100 (or even better, over 1000) which
then gets reduced to below 100.

[kib]

So for mostly idle machines, this increases the spacing between ':', ',', and values. Perhaps doing it conditionally is worse because the placement of the values would blink.