[MIPSEL][HPPA] Fix tests failing on quiet NaNs

[basilgello]

MIPSEL, MIPS64EL and HP-PA architectures have quiet and
signaling NaNs swapped. The test assumes correct values
for i386 / amd64 but obviously fails on these architectures.
It is documented in many places, like

https://software.llnl.gov/yorick-doc/stdlib/ieee.html :

Warning-- apparently there is no universal standard for what
constitutes signalling versus quiet NaN
on MIPS and HPPA architectures, qNaN and sNaN are reversed

https://gcc.gnu.org/onlinedocs/gcc/MIPS-Options.html :

-mnan=2008
-mnan=legacy

These options control the encoding of the special
not-a-number (NaN) IEEE 754 floating-point data.

The -mnan=legacy option selects the legacy encoding.
In this case quiet NaNs (qNaNs) are denoted by the first
bit of their trailing significand field being 0, whereas
signaling NaNs (sNaNs) are denoted by the first bit of
their trailing significand field being 1.

The -mnan=2008 option selects the IEEE 754-2008 encoding.
In this case qNaNs are denoted by the first bit of their
trailing significand field being 1, whereas sNaNs are
denoted by the first bit of their trailing significand
field being 0.

The default is -mnan=legacy unless GCC has been configured
with --with-nan=2008.

The behavior mentioned above causes FTBFS in Debian starting
with 1.12.0:

https://buildd.debian.org/status/fetch.php?pkg=flatbuffers&arch=mips64el&ver=1.12.1%7Egit20200711.33e2d80%2Bdfsg1-0.1&stamp=1594830681&raw=0
https://buildd.debian.org/status/fetch.php?pkg=flatbuffers&arch=mipsel&ver=1.12.1%7Egit20200711.33e2d80%2Bdfsg1-0.1&stamp=1594832647&raw=0
https://buildd.debian.org/status/fetch.php?pkg=flatbuffers&arch=hppa&ver=1.12.1%7Egit20200711.33e2d80%2Bdfsg1-0.1&stamp=1594830726&raw=0

For HP-PA (PA-RISC) there is no way to override the reversal,
and for MIPSEL/MIPS64EL there is no macro defining the '-mnan'
choice, so both variants were added with OR boolean operator.

[basilgello]

@aardappel @vglavnyy This issue was introduced in #5428 I tested the fix by building flatbuffers with GCC 8.3.0-4 mipsel and mips64el

[aardappel]

LGTM.. @vglavnyy ?

[vglavnyy]

@basilgello Thank you for the fix.
Platform-independent floating-point code is a real pain even on x64 Win/Nix.
LGTM

[basilgello]

@aardappel Please merge it then and please tag 1.12.1 release so I can get rid of ugly git prefixes in Debian :) Thanks!

[aardappel]

@basilgello I generally don't do releases for fixes unless its an absolute emergency (since it's a lot of work).

If you mean just adding a git tag.. I could, but that would confuse users of other languages/systems since there is no corresponding release.

I think living with uglyness is the easiest solution there :)

[basilgello]

OK, thanks for clarification! No problem! :)