RFR(XL): 8185640: Thread-local handshakes

[Andrew Haley]

This is a bad way to handle supports_thread_local_poll():

  static bool supports_thread_local_poll() {
#if defined(AMD64) || defined(SPARC)
    return true;
#else
    return false;
#endif
  }

Instead, it is better to use a flag which is #defined in the back
ends, and allow each back end to specify if it supports thread-local
handshakes.  We have *two* AARCH64 back ends, and only one of them
supports thread-local handshakes; both of them #define AARCH64.

#if defined(BLAH) should be reserved for hardware-specific properties,
not back-end-specific properties.

-- 
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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