RFR: 8315801: [PPC64] JNI code should be more similar to the Panama implementation [v2]

[Martin Doerr]

> This PR removes writing parameters to stack which are passed in registers. (See JBS issue for more details.) This allows us to get rid of the extra parameter for `c_calling_convention`.
> 
> We can also remove the extra space for the Parameter Save Area in some ABIv2 cases (ppc64le).
> 
> Tests have passed on linux ppc64 and ppc64le and AIX.

Martin Doerr has updated the pull request incrementally with one additional commit since the last revision:

  Further simplifications and improvements.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15599/files
  - new: https://git.openjdk.org/jdk/pull/15599/files/765aca11..4ab5372f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15599&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15599&range=00-01

  Stats: 45 lines in 2 files changed: 9 ins; 26 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/15599.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15599/head:pull/15599

PR: https://git.openjdk.org/jdk/pull/15599