runtime: move roots' bases calculation to gcMarkRootPrepare

This patch provides changes according to Austin's TODO. It just moves
calculation of base indexes of each root type from markroot function
to gcMarkRootPrepare.

Change-Id: Ib231de34e7f81e922762fc3ee2b1830921c0c7cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/279461


Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Austin Clements <austin@google.com>