RFR: 8278885: Remove Windows ARM64 int8_t workaround in G1

[aamarsh]

Please review this small change to revert a previous workaround that avoided an MSVC issue. 

This issue has now been fixed in VS 16.8 and higher. 

I also updated some documentation to reflect this change.

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Commit messages:
 - reverted windows/aarch64 workaround and added to documentation

Changes: https://git.openjdk.java.net/jdk/pull/6872/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6872&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278885
  Stats: 12 lines in 4 files changed: 2 ins; 6 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6872.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6872/head:pull/6872

PR: https://git.openjdk.java.net/jdk/pull/6872