RFR: JDK-8320368: Per-CPU optimization of Klass range reservation

[Thomas Stuefe]

In `Metaspace::reserve_address_space_for_compressed_classes`, we reserve space for the future Klass range. We place the Klass range somewhere that allows us to use "good" narrow Klass decoding later when initializing the encoding scheme.

Narrow Klass decoding is inherently CPU-specific, so doing this in shared coding is awkward. It leads to many ifdefs, vague code comments that are difficult to explain, and missed optimizations. 

There are common patterns: 
- all platforms benefit from unscaled encoding so trying to reserve <4GB for CDS=off is worthwhile. 

But there are more differences than one would think:
- some platforms (s390, riscv) benefit from reservation < 4GB even with CDS=on since a 32-bit immediate requires fewer instructions
- some platforms (aarch64) don't benefit from zero-based encoding, so no need to try that
- some platforms benefit from optimizing the base for 16-bit moves (PPC, s390, aarch64) or for other immediate formats (riscv)

It would be much better to have this section per CPU so that every CPU can implement its perfect, well documented version. A bit of code duplication is a good price for code clarity.

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This patch splits out `Metaspace::reserve_address_space_for_compressed_classes` into five variants, one per CPU (moving the code to CompressedKlassPointers); it also splits out `CompressedKlassPointers::initialize` into two variants, one for aarch64, one for all other platforms. 

Changes per-CPU:

#### aarch64:

Don't attempt to reserve for zero-based encoding; since lsl is not faster than movk. We reserve for movk mode right away if reserve for unscaled fails or if CDS=on.

We also add a last-ditch attempt to reserve optimized for movk via over-alignment. We only do this on aarch64 to prevent errors like this one JDK-8318119: "Invalid narrow Klass base on aarch64 post 8312018"

Since we don't want zero-based encoding, we need an aarch64-specific version for `CompressedKlassPointers::initialize()`

#### riscv:

We attempt to reserve at a "good" base that has only bits set either in [12..32), [32, 44) or in [44, 64).

#### s390:

We attempt to allocate < 4GB unconditionally.

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Depends on: https://git.openjdk.org/jdk/pull/16727

Commit messages:
 - Update compressedKlass_aarch64.cpp
 - JDK-8320368-Per-CPU-optimization-of-Klass-range-reservation

Changes: https://git.openjdk.org/jdk/pull/16743/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16743&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320368
  Stats: 466 lines in 14 files changed: 356 ins; 63 del; 47 mod
  Patch: https://git.openjdk.org/jdk/pull/16743.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16743/head:pull/16743

PR: https://git.openjdk.org/jdk/pull/16743