tools include compiler-gcc: Add __pure attribute helper

Adopt it from the kernel sources, will be used soon.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-oubheiqj8edo5rzewt11cbn0@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/include/linux/compiler-gcc.h

@@ -21,6 +21,9 @@
 /* &a[0] degrades to a pointer: a different type from an array */
 #define __must_be_array(a)	BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
 
+#ifndef __pure
+#define  __pure		__attribute__((pure))
+#endif
 #define  noinline	__attribute__((noinline))
 #ifndef __packed
 #define __packed	__attribute__((packed))