cmd/go/build: Add magic data for Plan 9 binaries.
cmd/go/build: Add magic data for Plan 9 binaries.

This change allows the Go build and install tools to
recognize Plan 9 i386 and amd64 binaries.

R=rsc, r, rminnich
CC=golang-dev
http://codereview.appspot.com/6575064

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,gnanderson/go,gnanderson/go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,tv42/old-go,apparentlymart/golang,dsriram/golang,tv42/old-go,gnanderson/go,dsriram/golang,apparentlymart/golang,dsriram/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,dsriram/golang