Make it an #error if we failed to detect the ARM architecture version

... or if it is less than ARMv5. The last ARMv4 Qt supported was Windows
CE 7, which was dropped for Qt 5.7 alongside MSVC 2008.

Change-Id: Ifc817705441a4aab9469ffff141dcfe491464efa
Reviewed-by: Maurice Kalinowski <maurice.kalinowski@qt.io>

src/corelib/global/qprocessordetection.h

@@ -141,6 +141,8 @@
 #  endif
 #  if Q_PROCESSOR_ARM >= 5
 #    define Q_PROCESSOR_ARM_V5
+#  else
+#    error "ARM architecture too old"
 #  endif
 #  if defined(__ARMEL__)
 #    define Q_BYTE_ORDER Q_LITTLE_ENDIAN