[jdk11u-dev] RFR: 8199138: Add RISC-V support to Zero

[Aleksey Shipilev]

Clean backport to enable Zero RISC64 builds in 11u.

Additional testing:
 - [x] Cross-compiling Linux Zero RISC64 works
 - [x] Resulting binaries run on HiFive Unmatched without problems

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Commit messages:
 - Backport edc4ffe54b8c1f042e98c0ddb9dce757d88b01f4

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/476/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=476&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8199138
  Stats: 15 lines in 2 files changed: 14 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/476.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/476/head:pull/476

PR: https://git.openjdk.java.net/jdk11u-dev/pull/476