gallivm: fix issues with trunc/round/floor/ceil with no arch rounding

The emulation of these if there's no rounding instruction available
is a bit more complicated than what the code did.
In particular, doing fp-to-int/int-to-fp will not work if the exponent
is large enough (and with NaNs, Infs). Hence such values need to be filtered
out and the original value returned in this case (which fortunately should
always be exact). This comes at the expense of performance (if your cpu
doesn't support rounding instructions).
Furthermore, floor/ifloor/ceil/iceil were affected by precision issues for
values near negative (for floor) or positive (for ceil) zero, fix that as well
(fixing this issue might not actually be slower except for ceil/iceil if the
type is not signed which is probably rare - note iceil has no callers left
in any case).

Also add some new rounding test values in lp_test_arit to actually test
for that stuff (which previously would have failed without sse41).

This fixes https://bugs.freedesktop.org/show_bug.cgi?id=59701.

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