gc, ld: tag data as no-pointers and allocate in separate section

gc, ld: tag data as no-pointers and allocate in separate section

The garbage collector can avoid scanning this section, with
reduces collection time as well as the number of false positives.
Helps a little bit with issue 909, but certainly does not solve it.

R=ken2
CC=golang-dev
https://golang.org/cl/5671099