RFR: Loom: RISCV stubs

[Fei Yang]

RISCV64 port has been upstreamed recently. This makes compile on Linux RISCV64 platform.
Note current patch does not implement Loom on this platform, but merely provides the absolute basic build capabilities.

Additional testing:
Linux RISCV64 cross-compilation

-------------

Commit messages:
 - RISCV stubs

Changes: https://git.openjdk.java.net/loom/pull/150/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=150&range=00
  Stats: 850 lines in 15 files changed: 722 ins; 112 del; 16 mod
  Patch: https://git.openjdk.java.net/loom/pull/150.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/150/head:pull/150

PR: https://git.openjdk.java.net/loom/pull/150