RFR: JDK-8300912: Update java/nio/MappedByteBuffer/PmemTest.java to run on x86_64 only

[Bill Huang]

This task is created to update jtreg test to run on x86 only because pmem emulation is only supported on x86. For other architectures with external NVRAM, run test with java PmemTest.

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Commit messages:
 - JDK-8300912 Update java/nio/MappedByteBuffer/PmemTest.java to run on x86_64 only

Changes: https://git.openjdk.org/jdk/pull/12352/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12352&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300912
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12352.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12352/head:pull/12352

PR: https://git.openjdk.org/jdk/pull/12352