Binary data in mprof.goc may prevent the garbage collector from freeing
memory blocks. This patch replaces all calls to runtime·mallocgc() with
calls to an allocator private to mprof.goc, thus making the private
memory invisible to the garbage collector. The addrhash variable is
moved outside of the .bss section.

R=golang-dev, dvyukov, rsc, minux.ma
CC=dave, golang-dev, remyoudompheng
https://golang.org/cl/7135063