cmd/link: replace golden binary files with hex dumps

cmd/link: replace golden binary files with hex dumps

The hex dumps will diff better, and I hope they will avoid
a repeat of http://bugs.debian.org/716853.

The CL will probably show the testdata diffs as "binary",
but in fact the binary versions are being replaced by
textual hex dumps (output of hexdump -C).

R=iant
CC=golang-codereviews
https://golang.org/cl/51000044