cmd/go: avoid use of 'go tool pack'
cmd/go: avoid use of 'go tool pack'

All packages now use the -pack option to the compiler.
For a pure Go package, that's enough.
For a package with additional C and assembly files, the extra
archive entries can be added directly (by concatenation)
instead of by invoking go tool pack.

These changes make it possible to rewrite cmd/pack in Go.

R=iant, r
CC=golang-dev
https://codereview.appspot.com/42910043

bryanxu/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,webfd/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,d0f/go-zh,d0f/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,d0f/go-zh,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,webfd/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,webfd/go-zh