Failures for s390x VXE2 with qemu

[blapie]

PR #477 pointed an issue with VXE2 with qemu.
Tests are currently disabled, thus limiting support for s390x.
Need to fix this!

[blapie]

Note that Jenkinsfile enabled test with the following options:

• GCC: only VXE enforced https://github.com/shibatch/sleef/blob/master/Jenkinsfile#L52
• CLANG-10: VXE2 also enforced but only in Debug mode https://github.com/shibatch/sleef/blob/master/Jenkinsfile#L70

[luhenry]

For GitHub actions, we only do Release builds, thus we hit the error. I would guess that LLVM doesn't generate the problematic instruction when in Debug mode.