cmd/compile: cleanup -dynlink/-shared support check
cmd/compile: cleanup -dynlink/-shared support check

Moves the list of architectures that support shared libraries into
a function. Also adds s390x to that list.

Change-Id: I99c8a9f6cd4816ce3d53abaabaf8d002e25e6b28
Reviewed-on: https://go-review.googlesource.com/21661
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Reviewed-by: Michael Hudson-Doyle <5e87ffb38a52af6fb9a19b679e9122c7a4d601a5@canonical.com>
Run-TryBot: Michael Munday <8f145917441a88646319703b14b4bef730f48765@ca.ibm.com>

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