cpu-exec: Allow "-d exec" in non-debug builds (drop CONFIG_DEBUG_EXEC)

The CONFIG_DEBUG_EXEC define compiles out a single qemu_log_mask()
call, which is a pretty trivial cost even for something in the main
cpu_exec() loop.  Having this be conditionally defined means that
'-d exec' on a non-debug build will silently do nothing.  Drop the
define and the configure machinery that sets it, in favour of just
always allowing this log option to be enabled at runtime.  As a
concession to the mainloopiness, we use qemu_loglevel_mask()+qemu_log()
rather than qemu_log_mask() to avoid the function call overhead.

Note that DEBUG_DISAS is always defined, so removing the
'|| defined(CONFIG_DEBUG_EXEC)' from those conditionals makes
no behavioural change for that logging.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
