RFR: vthreads don't support the reserved stack zone

[Dean Long]

vthreads don't support the reserved stack zone

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

Commit messages:
 - vthreads don't support the reserved stack zone

Changes: https://git.openjdk.java.net/loom/pull/120/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=120&range=00
  Stats: 12 lines in 3 files changed: 11 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/loom/pull/120.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/120/head:pull/120

PR: https://git.openjdk.java.net/loom/pull/120