RFR: 8339335: set number of parallel jobs when building webkit

[Johan Vos]

When building webkit, the perl script in `modules/javafx.web/src/main/native/Tools/Scripts/build-webkit` will check for a `-j` argument in `$makeArgs`. If that doesn't exist, it will default to `numberOfCPUs()`.
This PR passes the `NUM_COMPILE_THREADS` from `build.gradle` to the `build-webkit` script. When not setting `NUM_COMPILE_THREADS` explicitly,it will default to the number of CPU's as well, hence it will have the same behavior as without this PR.
When setting `NUM_COMPILE_THREADS` explicitly, the provided number will be used by cmake as well now.

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Commit messages:
 - Provide NUM_COMPILE_THREADS to webkit build, allowing to control the number of

Changes: https://git.openjdk.org/jfx/pull/1554/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1554&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339335
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1554.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1554/head:pull/1554

PR: https://git.openjdk.org/jfx/pull/1554