RFR: 8268296: ScopedMemoryAccess build error with readonly filesystems

[Liam Miller-Cushon]

This change fixes a build error during the generation of `ScopedMemoryAccess.java` when the sources are readonly, by using `cat a > b` instead of `cp a b` to avoid propagating the permissions to the generated source.

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Commit messages:
 - 8268296: ScopedMemoryAccess build error with readonly filesystems

Changes: https://git.openjdk.java.net/jdk/pull/4380/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4380&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268296
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4380.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4380/head:pull/4380

PR: https://git.openjdk.java.net/jdk/pull/4380