mpc: make it a proper package

* Convert mpc to a proper autotargets package
* Hook it up for binutils/gcc to use

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>