RFR: 8265031: Change default macOS min version for x86_64 to 10.12 and aarch64 to 11.0

[Kevin Rushforth]

As noted in the JBS bug, the minimum macOS version on which JavaFX will run is currently set to 10.10 in `mac.gradle`. macOS 10.10 is many years out of support, so we should update this minimum. Further, macOS / aarch64 (aka arm64) requires macOS 11.0 as a minimum in order to run.

The JDK recently updated their minimum for x86_64 to 10.12 and for aarch64 to 11.0. See [make/autoconf/flags.m4#L136](https://github.com/openjdk/jdk/blob/627ad9fe22a153410c14d0b2061bb7dee2c300af/make/autoconf/flags.m4#L136).

This PR will do the same for JavaFX.

To determine whether we are building on `aarch64`, I check the `TARGET_ARCH` property for either `aarch64` or `arm64`, since we currently use the latter for our builds, although we should proably update our build to consistently use `aarch64` everywhere, and map that to `arm64` on usage where required by native tools.

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Commit messages:
 - 8265031: Change default macOS min version for x86_64 to 10.12 and aarch64 to 11.0

Changes: https://git.openjdk.java.net/jfx/pull/462/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=462&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265031
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/462.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/462/head:pull/462

PR: https://git.openjdk.java.net/jfx/pull/462