[11u] RFR (S) 8255625: AArch64: Implement Base64.encodeBlock accelerator/intrinsic

[Dmitry Chuyko]

Hello,

Original RFE: https://bugs.openjdk.java.net/browse/JDK-8255625

The patch does not apply cleanly: corrected fuzz, adjusted small part to 
missing context in stubGenerator_aarch64.cpp (JDK-8224974 is not in 
11u). Resulting patch was formatted for 11u.

CheckGraalIntrinsics test already has this intrinsic.

11u webrev: http://cr.openjdk.java.net/~dchuyko/8255625/webrev.11u.00/

Testing: test/jdk/java/util/Base64, tier1, tier2.

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Thanks,
-Dmitry