qemu ARM host support likely to be broken in 1.0

This is just a quick email to summarise a discussion on IRC.

QEMU on ARM hosts (not ARM guests!) is currently broken in
at least the following ways:
 * configure will select the ucontext coroutine implementation
but on ARM makecontext returns ENOSYS and we will abort() on
startup
 * the fixed address we mmap the code gen buffer to in system
mode tends to overlap the libc heap (at least on Ubuntu oneiric);
when this happens you typically get a sysmalloc abort
 * there is a bug I have not investigated which causes an
i386 guest image to loop infinitely resetting before it brings
up the grub menu
 * the TCG_AREG0 is r7, which clashes with the Thumb frame
pointer, with the effect that --enable-debug fails to compile

ARM host support for TCG is not a very high priority for me and
I don't expect to fix any of these before 1.0 is released. I'm
happy to test patches if anybody else wishes to write them 

-- PMM