RFR: 8292816: GPL Classpath exception missing from assemblyprefix.h

[Tyler Steele]

Adds Classpath exception to assemblyprefix.h. Affects copyright header only.

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Commit messages:
 - Adds classpath excemption to assemblyprefix.h

Changes: https://git.openjdk.org/jdk/pull/9990/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9990&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292816
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9990.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9990/head:pull/9990

PR: https://git.openjdk.org/jdk/pull/9990