Fix definition of INFINITY and add NAN/HUGE_VAL[F]

v3: change __builtin_nanf() to __builtin_nanf("")
    This doesn't work yet, but it was agreed to commit as-is with the logic
    that "broken" is better than "completely missing" and this should be
    fixed in clang.

v2: use __builtin_inff() and also add nan/huge_val definitions

Signed-off-by: Aaron Watry <awatry@gmail.com>