fuzz/x86_emulate: Add 'afl-cov' target

fuzz/x86_emulate: Add 'afl-cov' target

...to generate a "normal" coverage-instrumented binary, suitable for
use with gcov or afl-cov.

This is slightly annoying because:

 - Every object file needs to have been instrumented to work
   effectively

 - You generally want to have both an afl-instrumented binary and a
   gcov-instrumented binary at the same time, but

 - gcov instrumentation and afl instrumentation are mutually exclusive

So when making the `afl-cov` target, generate a second set of object
files and a second binary with the `-cov` suffix.

While we're here, remove the redundant x86-emulate.c dependency for
x86-emulate.o.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>