CMake: Fix missing output of TEST_x86intrin_OUTPUT config test

When the intrinsics test failed, we never showed the failing build
output due to two reasons:
- TEST_x86intrin_OUTPUT was empty
- bracket arguments don't do variable expansion

Use the newly introduced feature in qt_config_compile_test to get
the output.
Replace the usage of a bracket argument with a concatenation of
regular strings.

Amends db342f42a4

Pick-to: 6.5 6.6 6.7
Task-number: QTBUG-122596
Change-Id: I7cdef9a145ac64c8fced8add4879fa19b8bcd19d
Reviewed-by:  Alexey Edelev <alexey.edelev@qt.io>

configure.cmake

@@ -1264,16 +1264,17 @@ https://github.com/llvm/llvm-project/issues/53520
 ]=]
         )
     else()
+        string(CONCAT error_message
+            "x86 intrinsics support missing. Check your compiler settings.\n"
+            "If this is an error, report at https://bugreports.qt.io with your compiler ID and "
+            "version, and this output:\n"
+            "\n"
+            "${TEST_x86intrin_OUTPUT}"
+        )
         qt_configure_add_report_entry(
             TYPE ERROR
             CONDITION (NOT QT_FEATURE_x86intrin)
-            MESSAGE [========[
-x86 intrinsics support missing. Check your compiler settings. If this is an
-error, report at https://bugreports.qt.io with your compiler ID and version,
-and this output:
-
-${TEST_x86intrin_OUTPUT}
-]========]
+            MESSAGE "${error_message}"
         )
     endif()
 endif()