Reliably generate positive or negative NaN again

...after e5c80bb69a30dfb0a3daf6061ab127d92f8142d6 "Purge out setNan from
math.cxx" had dropped the use of rtl::math::setNan and sign bit fiddling, and
relied on the implicit assumption that std::numeric_limits<double>::quiet_NaN
would produce a positive NaN (but which does not seem to be guaranteed by the
C++ standard) and on the expressed hope that multiplying such a positive NaN by
-1 would generate a negative NaN (but which does not seem to be guaranteed by
IEEE 754: while it mandates that a NaN's payload is preserved across such an
operation, the result's sign bit appears to be unspecified)

Change-Id: I12215c888a1cb8de6b3f046a836c550cb21b5a85
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/118604
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

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