compress/zlib: add Reset method to Writer.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13171046
cmd/ld: emit relocations for .debug_frame in external link mode

This should have been part of revision 16731:cdedb129e020, but
I missed it.  This fixes printing local variables when doing
an external link.

No test because we aren't doing any debug info testing yet.

Fixes issue 5719.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/13464046