arm64: Implement __clear_cache.

__clear_cache is defined in lib-arm64.c with a single call to
__arm64_clear_cache, which is the real built-in function and is
turned into inline assembler by gen_clear_cache in arm64-gen.c

lib/lib-arm64.c

@@ -14,7 +14,7 @@
 
 void __clear_cache(char *beg, char *end)
 {
-#warning __clear_cache not yet implemented
+    __arm64_clear_cache(beg, end);
 }
 
 typedef struct {