RFR: 8254846 Warn before building if Xcode has no disk space left

[George Adams]

This PR creates a simple sanity check for all systems to verify that there is at least 6 GB available disk space before compiling.

I got the 6 GB requirement from https://github.com/openjdk/jdk/blob/master/doc/building.md#building-on-x86

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Commit messages:
 - ensure that system has enough disk space before building

Changes: https://git.openjdk.java.net/jdk/pull/3693/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3693&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254846
  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3693.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3693/head:pull/3693

PR: https://git.openjdk.java.net/jdk/pull/3693