implement syncInstructionCache for ARM

Like, PowerPC, ARM has an instruction cache which must be manually
flushed if/when we compile a new method.  This commit updates
syncInstructionCache to use GCC's builtin __clear_cache routine.

src/arm.h

@@ -55,9 +55,11 @@ loadMemoryBarrier()
 }
 
 inline void
-syncInstructionCache(const void* start UNUSED, unsigned size UNUSED)
+syncInstructionCache(const void* start, unsigned size)
 {
-  asm("nop");
+  __clear_cache
+    (const_cast<void*>(start),
+     const_cast<uint8_t*>(static_cast<const uint8_t*>(start) + size));
 }
 
 typedef int (__kernel_cmpxchg_t)(int oldval, int newval, int *ptr);