testsuite: Ensure T5423 flushes C output buffer

Previously T5423 would fail to flush the printf output buffer.
Consequently it was platform-dependent whether the C or Haskell print
output would be emitted first.

testsuite/tests/rts/Makefile

@@ -37,7 +37,8 @@ T5423:
 	$(RM) T5423_cmm.o T5423.o T5423.hi T5423$(exeext)
 	"$(TEST_HC)" $(TEST_HC_OPTS) -v0 -c T5423_cmm.cmm
 	"$(TEST_HC)" $(TEST_HC_OPTS) -v0 -c T5423.hs
-	"$(TEST_HC)" $(TEST_HC_OPTS) -v0 T5423.o T5423_cmm.o -o T5423$(exeext)
+	"$(TEST_HC)" $(TEST_HC_OPTS) -v0 -c T5423_c.c
+	"$(TEST_HC)" $(TEST_HC_OPTS) -v0 T5423.o T5423_cmm.o T5423_c.o -o T5423$(exeext)
 	./T5423
 
 .PHONY: T9405

testsuite/tests/rts/T5423.hs

@@ -7,8 +7,13 @@ foreign import prim "test" test :: Int# -> Int# -> Int# -> Int# -> Int#
                                 -> Int# -> Int# -> Int# -> Int# -> Int#
                                 -> Int#
 
+foreign import ccall "flush_stdout" flush_stdout :: IO ()
+
 v :: Int
 v = I# (test 111# 112# 113# 114# 115# 116# 117# 118# 119# 120#)
 
 main :: IO ()
-main = print v
+main = do
+  n <- return $! v
+  flush_stdout -- Ensure that libc output buffer is flushed
+  print n

testsuite/tests/rts/T5423.stdout

-120
 111  112  113  114  115  116  117  118  119  120
+120

testsuite/tests/rts/T5423_c.c

+#include <stdio.h>
+
+void flush_stdout(void)
+{
+    fflush(stdout);
+}