ld: add NOPTRBSS for large, pointer-free uninitialized data

ld: add NOPTRBSS for large, pointer-free uninitialized data
cc: add #pragma textflag to set it
runtime: mark mheap to go into noptr-bss.
        remove special case in garbage collector

Remove the ARM from.flag field created by CL 5687044.
The DUPOK flag was already in p->reg, so keep using that.

Otherwise test/nilptr.go creates a very large binary.
Should fix the arm build.
Diagnosed by minux.ma; replacement for CL 5690044.

R=golang-dev, minux.ma, r
CC=golang-dev
https://golang.org/cl/5686060