Tru64: Skip tests that for some reason grind Tru64 to a halt.

fold_grind and pat_psycho finishe but take several minutes as opposed
to other re tests which take seconds; uniprops grinds for even longer
but eventually runs out of memory (ulimit mem ~0.5GB) (failing)

There probably should be a more centralized/general way of doing
this with the core tests: either 'on this $^O, skip these tests'
(to avoid crowding the BEGIN of each test with $^O testing),
or a more generalized watchdog system (if this test takes more
than N sec, bail out -- probably should not be a hard failure as
such by default, given slow systems), or some combination thereof.
Tru64: NaN comparison quite broken.

Too much Microsoft compatibility?
Tru64 nan() function broken.

Exists in libm, but no proto or man page, probably for a reason.
Return values seem to be garbage, or at least not NaN.

Also add a todo for the nan() payload.
Tru64: Do not undef c99_isunordered.
Tru64: Floating-point rounding control.
Tru64: introduce PERL_SYS_FPU_INIT, use it.

In Tru64 the cc -ieee enables the IEEE math but disables traps.
We need to reenable the "invalid" trap because otherwise generation
of NaN values leaves the IEEE fp flags in bad state, leaving any further
fp ops behaving strangely (Inf + 1 resulting in zero, for example).