[Flang] Fix test case for AIX(big-endian) system for issuing an extra message. (#104792)

The 10-byte FP on big-endian system such as AIX is not represented as a
valid number in FE, so Flang issues an extra WARNING message as
`underflow on REAL(10) to REAL(4) conversion` for `_10` for such a
number.
This PR is to fix the test case.

Author: DanielCChen

flang/test/Semantics/kinds04_q16.f90

@@ -1,4 +1,5 @@
-! RUN: %python %S/test_errors.py %s %flang_fc1
+! RUN: not %flang_fc1 %s 2>%t.stderr
+! RUN: FileCheck %s --input-file=%t.stderr --check-prefixes=%if system-aix %{"PORTABILITY","WARNING","ERROR","AIX_WARNING"%} %else %{"PORTABILITY","WARNING","ERROR"%}
 ! C716 If both kind-param and exponent-letter appear, exponent-letter
 ! shall be E. (As an extension we also allow an exponent-letter which matches
 ! the kind-param)
@@ -15,10 +16,12 @@ subroutine s(var)
   !PORTABILITY: Explicit kind parameter together with non-'E' exponent letter is not standard
   real :: realvar4 = 4.0D6_8
   !WARNING: Explicit kind parameter on real constant disagrees with exponent letter 'q'
+  !AIX_WARNING: underflow on REAL(10) to REAL(4) conversion
   real :: realvar5 = 4.0Q6_10
   !PORTABILITY: Explicit kind parameter together with non-'E' exponent letter is not standard
   real :: realvar6 = 4.0Q6_16
   real :: realvar7 = 4.0E6_8
+  !AIX_WARNING: underflow on REAL(10) to REAL(4) conversion
   real :: realvar8 = 4.0E6_10
   real :: realvar9 = 4.0E6_16
   !ERROR: Unsupported REAL(KIND=32)
@@ -32,6 +35,7 @@ subroutine s(var)
   !PORTABILITY: Explicit kind parameter together with non-'E' exponent letter is not standard
   double precision :: doublevar5 = 4.0Q6_16
   double precision :: doublevar6 = 4.0E6_8
+  !AIX_WARNING: underflow on REAL(10) to REAL(8) conversion
   double precision :: doublevar7 = 4.0E6_10
   double precision :: doublevar8 = 4.0E6_16
   !ERROR: Unsupported REAL(KIND=32)