RFR: 8280593: [PPC64, S390] redundant allocation of MacroAssembler in StubGenerator ctor

[Richard Reingruber]

This cleanup removes a redundant allocation from the StubGenerator constructor. The base class' constructor does the very same.

The fix passed our CI testing.

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Commit messages:
 - 8280593: [PPC64, S390] redundant allocation of MacroAssembler in StubGenerator ctor

Changes: https://git.openjdk.java.net/jdk/pull/7215/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7215&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280593
  Stats: 5 lines in 2 files changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7215.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7215/head:pull/7215

PR: https://git.openjdk.java.net/jdk/pull/7215