fix buffer overflow in printf formatting of denormals with low bit set

empirically the overflow was an off-by-one, and it did not seem to be
overwriting meaningful data. rather than simply increasing the buffer
size by one, however, I have attempted to make the size obviously
correct in terms of bounds on the number of iterations for the loops
that fill the buffer. this still results in no more than a negligible
size increase of the buffer on the stack (6-7 32-bit slots) and is a
"safer" fix unless/until somebody wants to do the proof that a smaller
buffer would suffice.
fix incorrect rounding in printf floating point corner cases

the printf floating point formatting code contains an optimization to
avoid computing digits that will be thrown away by rounding at the
specified (or default) precision. while it was correctly retaining all
places up to the last decimal place to be printed, it was not
retaining enough precision to see the next nonzero decimal place in
all cases. this could cause incorrect rounding down in round-to-even
(default) rounding mode, for example, when printing 0.5+DBL_EPSILON
with "%.0f".

in the fix, LDBL_MANT_DIG/3 is a lazy (non-sharp) upper bound on the
number of zeros between any two nonzero decimal digits.
math.h: make __FLOAT_BITS and __DOUBLE_BITS C89

Remove non-constant aggregate initializer. (Still using long long, but
that is supported by ancient compilers without __extension__ anyway).