cmd/link: stop generating garbage in elfhash

cmd/link: stop generating garbage in elfhash

All callers already had strings. No need to generate byte slice copies
to work on bytes.

Performance not measured, but probably helps at least a bit.

Change-Id: Iec3230b69724fac68caae7aad46f2ce1504e82e5
Reviewed-on: https://go-review.googlesource.com/20136
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>