RFR: 8284949: riscv: Add Zero support for the 32-bit RISC-V architecture

[Feilong Jiang]

This patch adds Zero support for the 32-bit RISC-V architecture.

Additional tests:

- [x] Linux zero RISCV32 cross-compilation
- [x] Resulting binaries run on QEMU User mode without problems

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Commit messages:
 - Add support for 32-bit risc-v zero

Changes: https://git.openjdk.java.net/jdk/pull/8284/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8284&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284949
  Stats: 23 lines in 4 files changed: 20 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8284.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8284/head:pull/8284

PR: https://git.openjdk.java.net/jdk/pull/8284