stdio-common: avoid repeated regexp matches in tst-printf-format.awk

stdio-common: avoid repeated regexp matches in tst-printf-format.awk

Whether the value is an infinity, a NaN or zero does not change between
the conversions applied to it, but was determined again for each one.
Determine it where the value is read.

Also look for the '#' flag with index() before matching the expressions
that need it, and test the value first where both have to hold.

For the %f conversion for double, in the C locale, as the median of five
runs:

  x86_64, gawk 5.4.1    1.248s -> 1.184s
  x86_64, gawk 5.3.2    0.703s -> 0.708s
  alpha,  gawk 5.4.60    26.6s ->  25.8s

So this only helps with the regular expression engine that gawk 5.4
brought in; under 5.3.2 it is lost in the noise.  Output and exit status
are unchanged for the e, f and g conversions for double under both
gawk versions and both locales.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>