Use GASNet short message Request/Reply for AM_signal handler.
Use GASNet short message Request/Reply for AM_signal handler.

When using the AM_signal handler, we will always only need a single
64-bit value for the "done" location, so we can use the short message
versions of AM_Request/Reply.  Break this up into two 32-bit values
(as required by the GASNet short message interface).

Added a comment about why we didn't make the "done" variables volatile
even thought we are spinning on them, because I seem to wonder this
every time I go back into the code.

[Reviewed by gbt]



git-svn-id: 88467cb1fb04b8a755be7e1ee1026be4190196ef@19456 3a8e244f-b0f2-452b-bcba-4c88e055c3ca

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