The convention for writing something like "64 kB" is 64<<10, since
this is easier to read than 1<<16. Update gcBitsChunkBytes to follow
this convention.

Change-Id: I5b5a3f726dcf482051ba5b1814db247ff3b8bb2f
Reviewed-on: https://go-review.googlesource.com/23132Reviewed-by: Rick Hudson <rlh@golang.org>