tests: od: avoid outputting undefined floating point values

tests: od: avoid outputting undefined floating point values

od was seen to abort() on glibc on ia64 and m68k with the error:
Fatal glibc error: printf_fp.c:501 (__printf_fp_buffer_1):
assertion failed:
  cy == 1 || (p.frac[p.fracsize - 2] == 0 && p.frac[0] == 0)

* tests/od/od-multiple-t.sh: Avoid outputting long double floats
to avoid undefined behavior. 'float' and 'double' are standardized
by IEEE 754 (except on Linux/m68k) and don't have undefined values.