RFR: JDK-8269393: store/load order not preserved when handling memory pool due to weakly ordered memory architecture of aarch64

[Damon Fenacci]

# Issue
An intermittent _Memory Pool not found_ error has been noticed when running  a few tests (_vmTestbase/vm/mlvm/meth/stress/compiler/deoptimize/Test.java_, _vmTestbase/vm/mlvm/meth/stress/compiler/sequences/Test.java_) on _macosx_aarch64_ (production build) with non-segmented code cache.

## Origin
The issue originates from the fact that aarch64 architecture is a weakly ordered memory architecture, i.e. it _permits the observation and completion of memory accesses in a different order from the program order_.

More precisely: while calling `CodeHeapPool::get_memory_usage`, the `used` and `committed` variables are retrieved
https://github.com/openjdk/jdk/blob/138542de7889e8002df0e15a79e31d824c6a0473/src/hotspot/share/services/memoryPool.cpp#L181-L182
and these are computed based on different variables saved in memory in `CodeCache::allocate` (during `heap->allocate` and `heap->expand_by` to be precise) .https://github.com/openjdk/jdk/blob/138542de7889e8002df0e15a79e31d824c6a0473/src/hotspot/share/code/codeCache.cpp#L535-L537
The problem happens when first `heap->expand_by` gets called (which _increases_ `committed`) and then `heap->allocate` gets called in a second loop pass (which _increases_ `used`). Although stores in `CodeCache::allocate` happen in the this order, when reading from memory in `CodeHeapPool::get_memory_usage` it can happen that `used` has the newly computed value, while `committed` is still "old" (because of ARM’s weak memory order). This is a problem, since `committed` must be > than `used`.

# Solution

To avoid this situation we must assure that values used to calculate `committed` are actually saved before the values used to calculate `used` and that the opposite be true for reading. To enforce this we acquire a `CodeCache_lock` while reading `used` and `committed` in `CodeHeapPool::get_memory_usage` (which should actually be the convention when accessing CodeCache data).

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Commit messages:
 - Revert "JDK-8269393: force store/load order"
 - Revert "JDK-8269393: fix position of load barrier"
 - Revert "JDK-8269393: revert used line indentation to match original"
 - JDK-8269393: revert used line indentation to match original
 - JDK-8269393: fix position of load barrier
 - JDK-8269393: force store/load order
 - JDK-8269393: Some vmTestbase/vm/mlvm/meth/stress/compiler tests fail with 'MemoryPool not found'

Changes: https://git.openjdk.org/jdk/pull/15819/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15819&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8269393
  Stats: 7 lines in 1 file changed: 5 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15819.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15819/head:pull/15819

PR: https://git.openjdk.org/jdk/pull/15819