tests: Fix gen-typegen.awk on AIX.

With AIX 7.2 awk, assigning $0 in an END action does not update NF,
leaving it with its prior value from the last record:

  aix72% echo a b c | awk 'END { $0 = "x"; print NF; }'
  3

In the case of gen-typegen.awk, this does not result in any syntax
errors in the generated typegen.h file.  However, the resulting
randomdecl executable fails to produce any declarations with more
than one type specifier in any given specifier list.

Fortunately the randomdecl sanity test caught this problem, because the
_Imaginary and _Complex specifiers were not being generated (as these
are the only type specifiers that never appear by themselves).

Work around the problem by using split instead.