cmd/gc: Reset haspointers computation.  When converting from a
slice type to an array type, the haspointer-ness may change.
Before this change, we'd sometimes get types like [1]int marked
as having pointers.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13189044
runtime: rename FlagNoPointers to FlagNoScan.  Better represents
the use of the flag, especially for objects which actually do have
pointers but we don't want the GC to scan them.

R=golang-dev, cshapiro
CC=golang-dev
https://codereview.appspot.com/13181045