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

[Ludovic Henry]

On Mon, 14 Sep 2020 19:42:21 GMT, Anton Kozlov <akozlov at openjdk.org> wrote:

> 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

I've verified that this code works for the Windows-AArch64 port with minimal changes to the new
`vm_version_windows_aarch64.cpp` file.

-------------

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