Add the missing meson infrastructure bits for the new libdw
dependency. Model them after the existing capstone knobs.

Fixes: 7c10cb38 ("accel/tcg: Add debuginfo support")
Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230210005208.438142-1-iii@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>