RFR: 8248736: [aarch64] runtime/signal/TestSigpoll.java failed "fatal error: not an ldr (literal) instruction."

[Stuart Monteith]

Change mov to movptr in the nmethod entry barrier. movptr will generate a consistent number of mov/movk instructions that are necessary to consistently calculate the size of the nmethod barrier.

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Commit messages:
 - 8248736: Fix nmethod_entry_barrier alignment

Changes: https://git.openjdk.java.net/jdk/pull/1481/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1481&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248736
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1481.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1481/head:pull/1481

PR: https://git.openjdk.java.net/jdk/pull/1481