Simplify Unsafe.{load|store}Fence fallbacks by delegating to fullFence

Unsafe.{load|store}Fence fallbacks call to unsafe.cpp for OrderAccess::{acquire|release}Fence(). It seems too heavy-handed (useless?) to call to runtime for a single memory barrier. We can simplify the native Unsafe interface by falling back to fullFence when {load|store}Fence intrinsics are not available. This would be similar to what {loadLoad|storeStore}Fences already do.