RFR: 8275645: [JVMCI] avoid unaligned volatile reads on AArch64

[Doug Simon]

This PR updates c2v-readFieldValue to always do the field read with volatile semantics but without using a volatile read instruction directly to avoid platform specific issues with unaligned reads (e.g., an unaligned ldar on AArch64 causes a SIGBUS).

-------------

Commit messages:
 - avoid unaligned volatile reads in c2v_readFieldValue
 - expanded MemoryAccessProviderTest to test unaligned reads

Changes: https://git.openjdk.java.net/jdk/pull/6044/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6044&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275645
  Stats: 63 lines in 6 files changed: 41 ins; 1 del; 21 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6044.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6044/head:pull/6044

PR: https://git.openjdk.java.net/jdk/pull/6044