cmd/objdump: import x86 disassembler

The x86 disassembler lives in rsc.io/x86/x86asm for now.
We need to figure out what should live where in the long term,
but not before the 1.3 release.

The completed code reviews for the disassembler are at:
https://golang.org/cl/95350044
https://golang.org/cl/95300044
https://golang.org/cl/97100047
https://golang.org/cl/93110044
https://golang.org/cl/99000043
https://golang.org/cl/98990043

LGTM=crawshaw
R=crawshaw, jacek.masiulaniec
CC=golang-codereviews
https://golang.org/cl/92360043

src/cmd/objdump/Makefile

+x86.go: bundle
+	./bundle -p main -x x86_ rsc.io/x86/x86asm >x86.go
+
+bundle:
+	go build -o bundle code.google.com/p/rsc/cmd/bundle