When I ran the INFINITY config test several times I found that the _RWSTD_LDBL_SNAN_BITS values are different from time to time.
Below is the result of 5 runs of the INFINITY test:
I suppose that actually used 80 bits from 96 bits of long double (sizeof (long double) == 12) and the latest two bytes contains the garbage from stack.
I think we need fill by zero values the bits fields of the flt_bits, dbl_bits and ldbl_bits unions (i.e. in union ctor).