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On x86_64 we are using C argument registers for global registers in
the STG machine.  This is always going to be problematic when it comes
to making C calls from STG and compiling via C.  Prior to GCC 4.1.0
(approx) it was possible to just assign the argument expressions to
temporaries to avoid a clash.  Now, we need to add an extra dummy
function call as a barrier between the temporary assignments and the
actual call.  The dummy call is removed by the mangler.

I've removed -fno-code from Main to make it work
equally well with --make and -c.
I've also allowed it not to write hi files unless
-fwrite-iface is given.

We need to use GNUC3_ATTRIBUTE(used) to force gcc to keep the
definition of StgRunIsImplementedInAssembler() around.  In some cases
we had already made this function external rather than static to get
around the problem, but GNUC3_ATTRIBUTE(used) is a better fix.

addresses #492

I've removed -fno-code from Main to make it work
equally well with --make and -c.
I've also allowed it not to write hi files unless
-fwrite-iface is given.

fixes failures in yesterday's testsuite run

Yet another attempt at shutdown & interruption.  This one appears to
work better; ^C is more responsive in multi threaded / SMP, and I
fixed one case where the runtime wasn't responding to ^C at all.

This fixes another instance of a subtle SMP bug (see patch "really
nasty bug in SMP").

This is just an assertion, in effect: we should never enter a PAP, but
for convenience we previously attached the PAP apply code to the PAP
info table.  The problem with this was that it makes it harder to track
down bugs that result in entering a PAP...