RFR: 8275874: [JVMCI] only support aligned reads in c2v_readFieldValue [v3]

[Doug Simon]

> [JDK-8275645](https://bugs.openjdk.java.net/browse/JDK-8275645) resulted in loosing single-copy atomicity for reads in `c2v_readFieldValue`. This PR fixes that only doing aligned reads in `c2v_readFieldValue`.

Doug Simon has updated the pull request incrementally with one additional commit since the last revision:

  fixed spelling and grammar

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6109/files
  - new: https://git.openjdk.java.net/jdk/pull/6109/files/4ca42e72..722e2d20

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6109&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6109&range=01-02

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6109.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6109/head:pull/6109

PR: https://git.openjdk.java.net/jdk/pull/6109