RFR: 8275874: [JVMCI] use volatile accessors for all unaligned reads in c2v_readFieldValue

[Doug Simon]

[JDK-8275645](https://bugs.openjdk.java.net/browse/JDK-8275645), resulted in the loose of single-copy atomicity for reads in c2v_readFieldValue. This PR fixes that by using the `<type>_field_acquire` accessors for all aligned reads in c2v_readFieldValue and only using the `<type>_field` accessors for unaligned reads.

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Commit messages:
 - use <type>_field_acquire for aligned reads in c2v_readFieldValue

Changes: https://git.openjdk.java.net/jdk/pull/6109/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6109&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275874
  Stats: 30 lines in 1 file changed: 2 ins; 18 del; 10 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