RFR: JDK-8260518: Change default -mmacosx-version-min to 10.12

[Erik Joelsson]

To guarantee backwards compatible binaries on Macos, we use the option -mmacosx-version-min. This is currently set to 10.9, which is a really ancient version. I propose we bump this to 10.12, which is still a rather conservative old version (support ended in 2019).

The driving issue for bumping this now is the aarch64 port, where building for aarch64 requires the version min to be set to 11.0. Having a large gap between the target versions becomes problematic as we hit a lot of deprecation warnings in shared code. To be able to fix these deprecation warnings, we need a smaller version gap.

Just bumping us to 10.12 triggers warnings in libsplashscreen, so I will temporarily add "deprecated-declarations" to the list of disabled warnings there until they can be fixed in JDK-8260402.

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Commit messages:
 - Bump macosx version min/max to 10.12

Changes: https://git.openjdk.java.net/jdk/pull/2268/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2268&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260518
  Stats: 4 lines in 3 files changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2268.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2268/head:pull/2268

PR: https://git.openjdk.java.net/jdk/pull/2268