2.3403: fix json_atof on AIX without HAS_LONG_DOUBLE (powl in libm)

[cpan #88061] Improved patch to check for HAS_LONG_DOUBLE (i.e. no powl)
use HAVE_NO_POWL instead of defined(_AIX) && (!defined(HAS_LONG_DOUBLE) || AIX_WORKAROUND)