RFR: 8275874: [JVMCI] use volatile accessors for aligned reads in c2v_readFieldValue [v2]

[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 by using `<type>_field_acquire` accessors for all aligned reads and only using `<type>_field` accessors for unaligned reads.

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

  restrict c2v_readFieldValue to only perform aligned reads

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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/5ea7fedb..4ca42e72

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6109&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6109&range=00-01

  Stats: 33 lines in 5 files changed: 20 ins; 2 del; 11 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