Adding s390x to the list of architectures that support c-shared and c-archive.
Required adding load-time initialization (via _rt0_s390x_linux_lib) and adding s390x
to the c-shared and c-archive tests.

Change-Id: I75883b2891c310fe8ce7f08c27b06895c074e123
Reviewed-on: https://go-review.googlesource.com/74910Reviewed-by: Michael Munday <mike.munday@ibm.com>