RFR: 8277846: Implement fast-path for ASCII-compatible CharsetEncoders on ppc64

[Martin Doerr]

PPC64 port of 8274242: Implement fast-path for ASCII-compatible CharsetEncoders on x86
I moved the `encode_iso_array` implementation into `C2_MacroAssembler` and reused it for the new ASCII node. The algorithm is unchanged. We only need to change the mask because (non-extended) ASCII uses 7 bit (also see x86 implementation).

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Commit messages:
 - 8277846: Implement fast-path for ASCII-compatible CharsetEncoders on ppc64

Changes: https://git.openjdk.java.net/jdk/pull/6565/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6565&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277846
  Stats: 77 lines in 3 files changed: 46 ins; 14 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6565.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6565/head:pull/6565

PR: https://git.openjdk.java.net/jdk/pull/6565