misc/cgo/testcshared, misc/cgo/testshared: fix clang warnings and errors

Fix several warnings generated on the linux-amd64-clang builder
and make it clear to clang that -znow is a linker only flag.

Tested with

    env CC=clang-3.5 ./all.bash
    env CC=gcc-4.8 ./all.bash

Change-Id: I5ca7366ba8bf6221a36d25a2157dda4b4f3e16fa
Reviewed-on: https://go-review.googlesource.com/9523


Reviewed-by: Ian Lance Taylor <iant@golang.org>