RFR: 8255615: Zero: demote ZeroStack::abi_stack_available guarantee to assert

[Aleksey Shipilev]

It is currently guarantee(), which slows down release bits unnecessarily:

inline int ZeroStack::abi_stack_available(Thread *thread) const {
  guarantee(Thread::current() == thread, "should run in the same thread");

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Commit messages:
 - 8255615: Zero: demote ZeroStack::abi_stack_available Thread check to assert

Changes: https://git.openjdk.java.net/jdk/pull/943/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=943&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255615
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/943.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/943/head:pull/943

PR: https://git.openjdk.java.net/jdk/pull/943