POSIX math: Add the opengroup URL for math.h
NV_INF/NV_NAN fallback where int32 is coerced to IEEE754 float.

This is the second-to-last fallback.  The last one uses explicit
1/0.0 and 0/0.0 which may cause consternation with some compilers.
The .i target should depend at least on perl.h and config.h.

While at it, do the same for the .s target.
Long double might have INFINITYL or NANL.