Only the emulators link to code that uses capstone, so adding it to the
global LIBs places undesirable dependancies on other binaries, in
particular the tools.

There is no variable that covers both user emulation and machine
emulation, so add a new "$libs_cpu" for this purpose.

In particular this removes the 8 MB capstone dep from the things
qemu-img links against, allowing for a more minimal installation
in scenarios that don't want system emulators installed.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>