RFR: 8280964: [Linux aarch64] : drawImage dithers TYPE_BYTE_INDEXED images incorrectly

[Phil Race]

As per the bug evaluation, the root of this issue is that on this platform "char" is treated as "unsigned char"
and the code here expects it to be "signed char", so we need to make it explicitly so.
For all other supported platforms this is a no-op since that is the default elsewhere.
So this should be a safe fix, and the updated JDK has been tested on all platforms.

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Commit messages:
 - 8280964: [Linux aarch64] : ImageIO JPEGWriter writes TYPE_BYTE_INDEXED images incorrectly

Changes: https://git.openjdk.java.net/jdk/pull/7563/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7563&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280964
  Stats: 100 lines in 8 files changed: 83 ins; 0 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7563.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7563/head:pull/7563

PR: https://git.openjdk.java.net/jdk/pull/7563