[jdk18] RFR: 8279702: [macosx] ignore xcodebuild warnings on M1

[Johannes Bechberger]

Simple change to `make/autoconf/toolchain.m4` to fix the bug as described in its title and description.

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Commit messages:
 - Update copyright
 - Ignore the error output of xcodebuild when checking its version

Changes: https://git.openjdk.java.net/jdk18/pull/95/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk18&pr=95&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279702
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk18/pull/95.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk18 pull/95/head:pull/95

PR: https://git.openjdk.java.net/jdk18/pull/95