target-m68k: Remove custom qemu_assert() function

Remove the custom qemu_assert() function defined by target-m68k/translate.c
in favour of either using glib g_assert_not_reached() (for the genuinely
can't-happen cases) or cpu_abort() (for the "this isn't implemented",
in line with other unimplemented cases in the target).

This has the benefit of silencing some clang warnings about
variables used while uninitialized (which are emitted because
clang can't figure out that qemu_assert(0, something) never
returns.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
