cmd/5g, cmd/5l, cmd/ld: Small adjustments for the Plan 9 native tools

A few USED(xxx) additions and a couple of deletions of variable
initialisations that go unused.  One questionable correction,
mirrored in 8l/asm.c, where the result of invocation of a function
shouldn't be used.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/6736054

Committer: Russ Cox <rsc@golang.org>
cmd/5g, cmd/6g, cmd/8g: flush return parameters in case of panic.

Fixes issue 4066.

R=rsc, minux.ma
CC=golang-dev
https://codereview.appspot.com/7040044