gen-strtab.awk: Work around AIX substitution bug.

gen-strtab.awk: Work around AIX substitution bug.

On AIX 7.2 awk, the sub and gsub functions have a bug when the
replacement string contains a "\1" character.  The substituted
strings have \1 characters replaced with amperands, for example:

  aix72% awk 'BEGIN { s="x"; sub("x","\1",s); sub("\1","x",s); print s; }'
  &

Very strange.  The problem is internal to (g)sub, there is no
problem using "\1" characters in other contexts -- including
with the other arguments to (g)sub.  It should be fine to use
"\2" instead which avoids this particular bug.