Intrinsify Unsafe::setMemory

Unsafe::setMemory is for instance used to zero out native MemorySegments when they are allocated.

Currently it only has a native implementation in unsafe.cpp, meaning that we do 2 thread state transitions on every call (Java -> native and native -> VM).

By intrinsifying this method in C2 (as a leaf call), we could avoid both state transitions which should yield significant speedups for small fill sizes.