Produce IEEE compliant output from pow() despite platform deviations

src/cmd/ksh93/features/math.sh:
- Specify ast_float.h within iffehdrs instead of math.h, so that iffe
  will pick up on macro substitutions within libast. This should make
  any future efforts to remedy floating point behavior easier as well.
- Always include ast_float.h within the generated math header file,
  not just on IA64 platforms.

src/cmd/ksh93/tests/arith.sh:
- Test pow(1.0,-Inf) and pow(1.0,NaN) for IEEE compliance as well.
- Test the exponentiation operator (**) in addition, as streval.c,
  which processes the same, calls pow() separately.

src/lib/libast/features/float:
- Test the IEEE compliance of the underlying math library's pow()
  function and substitute macros producing compliant behavior if
  necessary.

src/cmd/ksh93/tests/arith.sh

@@ -297,6 +297,12 @@ fi
 if	(( (4**3)**2 != pow(pow(4,3),2) ))
 then	err_exit '(4**3)**2 not working'
 fi
+if	(( 1**Inf != pow(1,Inf) ))
+then	err_exit '1**Inf not working'
+fi
+if	(( 1**NaN != pow(1,NaN) ))
+then	err_exit '1**NaN not working'
+fi
 typeset -Z3 x=11
 typeset -i x
 if	(( x != 11 ))
@@ -465,8 +471,12 @@ then	set \
 	(( NaN != NaN )) || err_exit 'NaN == NaN'
 	(( -5*Inf == -Inf )) || err_exit '-5*Inf != -Inf'
 	[[ $(print -- $((sqrt(-1.0)))) == ?(-)nan ]]|| err_exit 'sqrt(-1.0) != NaN'
+	(( pow(1.0,-Inf) == 1.0 )) || err_exit 'pow(1.0,-Inf) != 1.0'
+	(( pow(-Inf,0.0) == 1.0 )) || err_exit 'pow(-Inf,0.0) != 1.0'
 	(( pow(1.0,Inf) == 1.0 )) || err_exit 'pow(1.0,Inf) != 1.0'
 	(( pow(Inf,0.0) == 1.0 )) || err_exit 'pow(Inf,0.0) != 1.0'
+	(( pow(1.0,NaN) == 1.0 )) || err_exit 'pow(1.0,NaN) != 1.0'
+	(( pow(Nan,0.0) == 1.0 )) || err_exit 'pow(Nan,0.0) != 1.0'
 	[[ $(print -- $((NaN/Inf))) == ?(-)nan ]] || err_exit 'NaN/Inf != NaN'
 	(( 4.0/Inf == 0.0 )) || err_exit '4.0/Inf != 0.0'
 else	err_exit 'Inf and NaN not working'