[jdk11u-dev] RFR: 8260518: Change default -mmacosx-version-min to 10.12

[Vladimir Kempik]

The backport is not applying clean due to the context code difference.
It's needed mostly to resolve this compilation error on macos-aarch64
splashscreen_sys.m:274:39: error: 'NSAlphaFirstBitmapFormat' is deprecated: first deprecated in macOS 10.14 [-Werror,-Wdeprecated-declarations]

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Commit messages:
 - 8260518: Change default -mmacosx-version-min to 10.12

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/810/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=810&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260518
  Stats: 4 lines in 3 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/810.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/810/head:pull/810

PR: https://git.openjdk.java.net/jdk11u-dev/pull/810