FYI: 8242185: Lanai: JDK built on macOS 10.15 fails to run on macOS 10.13

[Kevin Rushforth]

By default, the `metal` shader compiler generates code for the highest metal shader language version supported by the
SDK being used and the system on which it is compiled. So when building on macOS 10.15 (and maybe 10.14 but I didn't
try it), it will use metal shader version 2.1. The highest version supported on macOS 10.13 is shader version 2.0.

The simple fix is to specify the shader version when compiling the metal shader.

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Commit messages:
 - 8242185: Lanai: JDK built on macOS 10.15 fails to run on macOS 10.13

Changes: https://git.openjdk.java.net/lanai/pull/21/files
 Webrev: https://webrevs.openjdk.java.net/lanai/21/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8242185
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/lanai/pull/21.diff
  Fetch: git fetch https://git.openjdk.java.net/lanai pull/21/head:pull/21

PR: https://git.openjdk.java.net/lanai/pull/21