cputlb: don't cpu_abort() if guest tries to execute outside RAM or RAM

In get_page_addr_code(), if the guest program counter turns out not to
be in ROM or RAM, we can't handle executing from it, and we call
cpu_abort(). This results in the message
  qemu: fatal: Trying to execute code outside RAM or ROM at 0x08000000
followed by a guest register dump, and then QEMU dumps core.

This situation happens in one of two cases:
 (1) a guest kernel bug, where it jumped off into nowhere
 (2) a user command line mistake, where they tried to run an image for
     board A on a QEMU model of board B, or where they didn't provide
     an image at all, and QEMU executed through a ROM or RAM full of
     NOP instructions and then fell off the end

In either case, a core dump of QEMU itself is entirely useless, and
only confuses users into thinking that this is a bug in QEMU rather
than a bug in the guest or a problem with their command line. (This
is a variation on the general idea that we shouldn't assert() on
something the user can accidentally provoke.)

Replace the cpu_abort() with something that explains the situation
a bit better and exits QEMU without dumping core.

(See LP:1062220 for several examples of confused users.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson  <rth@twiddle.net>
Message-id: 1466442425-11885-1-git-send-email-peter.maydell@linaro.org

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