Add z15 support (#343)

This patch adds z15 VX support, following issue https://github.com/shibatch/sleef/issues/317

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It also changes the suffix zvector2 to vxe and vxe2.
In order to enable z15 VXE2 support, it requires clang-10.
However, it seems that clang-9 and later have a bug in handling orderedness of comparison.
Thus, it passes the tests only if optimizations are turned off.

Co-authored-by: shibatch <shibatch.sf.net@gmail.com>