Follow up to r1563971: Make the JavaHL JVM crasher function more robust
and safer on weird architectures that actually allow writing to the
zero page.

* subversion/bindings/javahl/native/JNIUtil.cpp
  (gentle_crash_write_loc): Change type to svn_atomic_t.
  (gently_crash_the_jvm): Use atomic operations to read the value at
   address 0 first, and if that actually works, write the same value
   back. This might cause a slight stutter on a system that maps its
   aircraft engine control registers to address 0.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1564252 13f79535-47bb-0310-9956-ffa450edef68