[arm] (header usage fix) include c++ algorithm header via system.h

After the recent commit that forces uses of c++11, the arm part failed
to build because it does not include <algorithm> via system.h as
should be done.

This results in:
from /gcc/common/config/arm/arm-common.c:34:
/usr/lib/gcc/x86_64-linux-gnu/5/include/mm_malloc.h:42:12: error:
attempt to use poisoned "malloc"
     return malloc (size);

This patch fixes the problem by defining INCLUDE_ALGORITHM before
including system.h and no longer includes <algorithm> directly.

gcc/ChangeLog:

	* common/config/arm/arm-common.c (INCLUDE_ALGORITHM):
	Define. No longer include <algorithm>.

gcc/common/config/arm/arm-common.c

@@ -19,6 +19,7 @@
 
 #define INCLUDE_LIST
 #define INCLUDE_VECTOR
+#define INCLUDE_ALGORITHM
 #include "config.h"
 #include "system.h"
 #include "coretypes.h"
@@ -31,7 +32,6 @@
 #include "flags.h"
 #include "sbitmap.h"
 #include "diagnostic.h"
-#include <algorithm>
 
 /* Set default optimization options.  */
 static const struct default_options arm_option_optimization_table[] =