PIE binaries can be built by the Go compiler in external link mode with
extldflags="-pie". These binaries support ASLR (address space layout
randomization) when executed on systems with appropriate kernel/dynamic
linker support.

This CL enables some cgo tests to run with -pie as a sanity check (in
addition to the other linker flag combinations they already test).

I have tested this functionality more thoroughly by building the full
compiler testsuite (test/...) and standard library tests with -pie
and executing them remotely on ChromeOS devices for all three linux
architectures (linux_amd64, linux_386, and linux_arm).

Change-Id: I3f644a72e94c3341f3360dfee58db5ec3a591e26
Reviewed-on: https://go-review.googlesource.com/6280Reviewed-by: Ian Lance Taylor <iant@golang.org>