cmd/dist: avoid use of 'go tool pack'
cmd/dist: 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/42890043

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