RFR: 8283929: GHA: Add RISC-V build config

Mon Sep 5 13:28:27 UTC 2022

It adds CI for cross-compiling to linux-riscv64. It requires to use Ubuntu repositories for the debootstrap image because Debian doesn't have riscv64 packages on https://httpredir.debian.org/debian/. We can follow up on this change with switching to http://ports.ubuntu.com/ubuntu-ports for all cross-compiled architectures.

>From talking with @shipilev, this replaces https://github.com/openjdk/jdk/pull/10084.

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Commit messages:
 - Add cross-compilation to riscv64 on CI

Changes: https://git.openjdk.org/jdk/pull/10168/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10168&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8283929
  Stats: 33 lines in 1 file changed: 25 ins; 5 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/10168.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10168/head:pull/10168

PR: https://git.openjdk.org/jdk/pull/10168