runtime: struct Obj in mgc0.c and buffers in scanblock()

Details:

- This CL is the conceptual skeleton of code found in CL 6114046

- The garbage collector uses struct Obj to specify memory blocks

- scanblock() is putting found memory blocks into an intermediate buffer
  (xbuf) before adding/flushing them to the main work buffer (wbuf)

- The main loop in scanblock() is replaced with a skeleton code that
  in the future will be able to recognize the type of objects and
  thus will improve the garbage collector's precision.
  For now, all objects are simply sequences of pointers so
  the precision of the garbage collector remains unchanged.

- The code plugs .gcdata and .gcbss sections into the garbage collector.
  scanblock() in this CL is unable to make any use of this.

R=rsc, dvyukov, remyoudompheng
CC=dave, golang-dev, minux.ma
https://golang.org/cl/6856121