tdf#143450: Fix special fp+integer struct return case for gcc_*_x86-64

tdf#143450: Fix special fp+integer struct return case for gcc_*_x86-64

For one, the loop in x86_64::fill_struct was backwards.

And for another, privateSnippedExecutor does not need special handling of FLOAT
and DOUBLE return values (they can simply be moved to %xmm0, as covered by the
general case), but rather for those small structs where floating-point member(s)
in a first eightbyte (targeting %xmm0) are followed by integer member(s) in a
second eightbyte (targeting %rax).  Extended testtools to cover two such cases.

Change-Id: I8e775a1d1ce2312610f265bcc8e40b09bdac56df
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/119576
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>