- add buffered stdout to all tools and provide to link ctxt.
- avoid extra \n before ! in .6 files written by assemblers
  (makes them match the C compilers).
- use linkwriteobj instead of linkouthist+linkwritefuncs.
- in assemblers and C compilers, record pc explicitly in Prog,
  for use by liblink.
- in C compilers, preserve jump target links.
- in Go compilers (gsubr.c) attach gotype directly to
  corresponding LSym* instead of rederiving from instruction stream.
- in Go compilers, emit just one definition for runtime.zerovalue
  from each compilation.

This CL consists entirely of small adjustments.
The heavy lifting is in CL 39680043.
Each depends on the other.

R=golang-dev, dave, iant
CC=golang-dev
https://golang.org/cl/37030045