format: Check for NaN when rendering doubles

format: Check for NaN when rendering doubles

- The result of 0.0/0.0 was being rendered as a lot of
  zeroes in the integer portion, as in this example:

  DBGvpp# show physmem
  0: 16 objects, 576k of 582k used, 3k free, 0 reclaimed, 2k overhead,
  16380k capacity
       alloc. from small object cache: 0 hits 0 attempts (0.00%) replacements 0
       alloc. from free-list: 0 attempts, 0 hits (0.00%), 0 considered (per-attempt 0.00)
       alloc. from vector-expand: 16
       allocs: 16 73643.06 clocks/call
       frees: 0 000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00 clocks/call

- Add two macros to vppinfra/math.h that use compiler builtins to check
  for NaN and Infinity and then use them in format_float().

Change-Id: Iccc03997e6e33d6b888d1e7e20cd78df0cfd02e8
Signed-off-by: Chris Luke <chrisy@flirble.org>