RFR: 8253015: Aarch64: Move linux code out from generic CPU feature detection

[Anton Kozlov]

Please review a change to remove Linux-specific code from hotspot/cpu/aarch64.

The change is made to prepare for non-linux aarch64 platforms.

vm_version no longer contains any "raw" values that were obtained directly from the platform registers. Some of them
may be unavailable on certain architectures, like `ctr_el0` is not available on windows [1].

Few opportunities to improve linux code were taken:
1. `HWCAP_` flags now explicitly mapped to `CPU_`
2. `_dcpop` boolean was replaced with `CPU_DCPOP`
3. Code generation to get the platform register values was replaced with inline assembly. There is no a code buffer
allocation anymore.

Testing:
* local hotspot tier1
* looking at detected features

[1] https://mail.openjdk.java.net/pipermail/aarch64-port-dev/2020-September/009690.html

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Commit messages:
 - Reduce includes
 - Move linux code from aarch64 feature detection

Changes: https://git.openjdk.java.net/jdk/pull/154/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=154&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253015
  Stats: 372 lines in 7 files changed: 163 ins; 153 del; 56 mod
  Patch: https://git.openjdk.java.net/jdk/pull/154.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/154/head:pull/154

PR: https://git.openjdk.java.net/jdk/pull/154