Correctly handle DW_FORM_flag and add DW_FORM_flag_present support

Before this patch, running pahole on an executable compiled
with -gdwarf-4 gave:

DW_AT_<0x3c>=0x19

0x19 is DW_FORM_flag_present.
Also, DW_FORM_flag takes an argument.
Changing attr_numeric to call dwarf_formflag fixes both problems.