RFR: Add s390(x) detection to platform.m4

[Omair Majid]

Hi,

Over in Fedora-land, we are trying to build OpenJDK (using zero) on
s390(x) [1]. One of the changes needed is updating the platform.m4 file
to teach it about s390(x). The following webrev does that:
http://cr.openjdk.java.net/~omajid/webrevs/s390-platform/00/

The patch was written by Dan Horák <dan at danny.cz> (who works for Red Hat
and is covered by the CLA).

Thanks,
Omair
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