RFR: 8294394: running jlink in GraalVM must copy libgraal into the output image

[Doug Simon]

This PR adds a new jlink plugin (`--copy-files=<filenames>`) that copies specified files from the current image into the output image.
This is useful in the context of GraalVM where libgraal (e.g. `lib/libjvmcicompiler.so`) is produced after the final jlink step in the GraalVM JDK build process. The plugin makes it possible for jlink in a GraalVM JDK to propagate libgraal.

The advantages of this solution over packaging libgraal as a module are:
* No need to complicate and slow down the GraalVM JDK build process with an extra jlink step.
* No need to pay the footprint of libgraal twice in a GraalVM JDK (i.e., once in,`lib/libjvmcicompiler.so` and again in a jmod file).

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Commit messages:
 - add CopyFilesPlugin

Changes: https://git.openjdk.org/jdk/pull/10448/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10448&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294394
  Stats: 205 lines in 4 files changed: 205 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10448.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10448/head:pull/10448

PR: https://git.openjdk.org/jdk/pull/10448