RFR: 8307955: Prefer to PTRACE_GETREGSET instead of PTRACE_GETREGS in method 'ps_proc.c::process_get_lwp_regs'

[Guoxiong Li]

Hi all,

This patch revises the code of `ps_proc.c::process_get_lwp_regs`
to use `PTRACE_GETREGSET` first instead of `PTRACE_GETREGS`.
The `PTRACE_GETREGS` is not present on all architectures as the man page states [1].
And if we use `PTRACE_GETREGS` first, several tests will fail at the special envs,
such as my local riscv64-linux env. Please see [the issue](https://bugs.openjdk.org/browse/JDK-8307955) for more information.

And I remove the unnecessary comments and macro,
because the `sparc` arch related code had been removed.

Thanks for the review.

Best Regards,
-- Guoxiong

[1] https://man7.org/linux/man-pages/man2/ptrace.2.html

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Commit messages:
 - JDK-8307955

Changes: https://git.openjdk.org/jdk/pull/13939/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13939&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307955
  Stats: 24 lines in 1 file changed: 9 ins; 13 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13939.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13939/head:pull/13939

PR: https://git.openjdk.org/jdk/pull/13939