RFR: 8253702: BigSur java/lang/System/OsVersionTest.java: 10.16 != 11.0

[Roger Riggs]

On Mac Os X, the OSVersionTest detected a difference in the version number reported in the os.version property
and the version number provided by `sw_vers -productVersion`.

When the java runtime is built with XCode 11.3, the os.version is reported as 10.16
though the current version numbering is 11.nnn.  

The workaround is to derive the os.version number from the ProductBuildVersion.
When the toolchain is updated to XCode 12.nnn it can be removed.
The workaround is enabled only when the environment variable SYSTEM_VERSION_COMPAT is unset.  
When the SYSTEM_VERSION_COMPAT is set in the environment the version number is reported as reported by Mac OS X.

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Commit messages:
 - 8253702: BigSur java/lang/System/OsVersionTest.java: 10.16 != 11.0

Changes: https://git.openjdk.java.net/jdk/pull/2530/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2530&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253702
  Stats: 30 lines in 1 file changed: 21 ins; 2 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2530.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2530/head:pull/2530

PR: https://git.openjdk.java.net/jdk/pull/2530