gh-158567: Workaround GCC crash in PyFloat_Pack2()

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Avoid __builtin_bswap16() with _Float16 on s390x.

• Issue: Optimize PyFloat_Pack*() functions using _Py_bswap16/32/64() functions #158567

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!buildbot s390x Fedora Stable LTO

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🤖 New build scheduled with the buildbot fleet by @vstinner for commit 3c23817 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F158568%2Fmerge

The command will test the builders whose names match following regular expression: s390x Fedora Stable LTO

The builders matched are:

• s390x Fedora Stable LTO + PGO PR
• s390x Fedora Stable LTO PR

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buildbot/s390x Fedora Stable LTO + PGO PR

Ok, good, the "compile" step completed successfully.

I also tested the change manually on the s390x Fedora Stable buildbot using: ./configure && make -j10 && ./python -m test test_float test_capi.

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Python built successfully on the two s390x Fedora Stable jobs (LTO and "LTO + PGO").