[lworld] RFR: 8315272: [lworld] Replacing NULL with nullptr in aarch64 code

[Frederic Parain]

NULL -> nullptr replacement in aarch64 code.

Tested with Mach5 tier1.

Fred

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

Commit messages:
 - Rewrite NULL to nullptr in aarch64 code

Changes: https://git.openjdk.org/valhalla/pull/920/files
 Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=920&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315272
  Stats: 47 lines in 6 files changed: 0 ins; 0 del; 47 mod
  Patch: https://git.openjdk.org/valhalla/pull/920.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/920/head:pull/920

PR: https://git.openjdk.org/valhalla/pull/920