runtime: don't mark unrolled bitmap type as having a gc program

runtime: don't mark unrolled bitmap type as having a gc program

It doesn't have a GC program - the whole point is that it is
the unrolled version of a GC program.

Fortunately, this isn't a bug as (*mspan).typePointersOfUnchecked
ignores the GCProg flag and just uses GCData as a bitmap unconditionally.

Change-Id: I2508af85af4a1806946e54c893120c5cc0cc3da3
Reviewed-on: https://go-review.googlesource.com/c/go/+/555355
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@google.com>