RFR: 8255339: CodeBuffer::_last_insn is only used on AArch64

[Eric Liu]

CodeBuffer::_last_insn was used to merge consecutive memory barriers and
adjacent load/store on AArch64.

This patch marks it as AArch64 only so that 8 bytes could be dropped
from CodeBuffer on other architectures.

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Commit messages:
 - 8255339: CodeBuffer::_last_insn is only used on AArch64

Changes: https://git.openjdk.java.net/jdk/pull/1971/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1971&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255339
  Stats: 5 lines in 1 file changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1971.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1971/head:pull/1971

PR: https://git.openjdk.java.net/jdk/pull/1971