Testsuite/Fortran: gfortran.dg/pr96711.f90 - fix expected value for PowerPC [PR96983]

gcc/testsuite/ChangeLog:

	PR fortran/96983
	* gfortran.dg/pr96711.f90: Use 2**digit(x) instead of a hard-coded value;
	add comments regarding what the code does.

diff --git a/gcc/testsuite/gfortran.dg/pr96711.f90 b/gcc/testsuite/gfortran.dg/pr96711.f90
index 3761a8ea416..ab7707d0120 100644
--- a/gcc/testsuite/gfortran.dg/pr96711.f90
+++ b/gcc/testsuite/gfortran.dg/pr96711.f90
@@ -1,28 +1,30 @@
 ! { dg-do run }
 ! { dg-require-effective-target fortran_integer_16 }
 ! { dg-require-effective-target fortran_real_16 }
 ! { dg-additional-options "-fdump-tree-original" }
 ! { dg-final { scan-tree-dump-times "_gfortran_stop_numeric" 2 "original" } }
 !
 ! PR fortran/96711 - ICE on NINT() Function
 
 program p
   implicit none
   real(8)                :: x
   real(16)               :: y
+  ! Assume radix(x) == 2
+  ! 2/epsilon(x) = 2/(radix(x)**(1-digits(x)) = 2**digits(x) with that assumption
   integer(16), parameter :: k1 = nint (2 / epsilon (x), kind(k1))
   integer(16), parameter :: k2 = nint (2 / epsilon (y), kind(k2))
-  integer(16), parameter :: m1 = 9007199254740992_16                    !2**53
-  integer(16), parameter :: m2 = 10384593717069655257060992658440192_16 !2**113
+  integer(16), parameter :: m1 = 2_16**digits(x) ! Usually 2**53
+  integer(16), parameter :: m2 = 2_16**digits(y) ! Might be 2**113 or 2**106 or ... depending on the system
   integer(16), volatile  :: m
   x = 2 / epsilon (x)
   y = 2 / epsilon (y)
   m = nint (x, kind(m))
 ! print *, m
   if (k1 /= m1) stop 1
   if (m  /= m1) stop 2
   m = nint (y, kind(m))
 ! print *, m
   if (k2 /= m2) stop 3
   if (m  /= m2) stop 4
 end program