gcc: introduce BR2_GCC_NEEDS_MPC to reduce code duplication

For each version of gcc, we need to check whether it requires mpc as a
dependency. Since this is true for 4.5, 4.6, 4.7, snapshots and now
4.8, let's factorize this code a bit by using a Kconfig symbol that
tells us whether we are using a gcc version that requires mpc.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>