[oe] Compiler flags in machine features for specific GCC versions

[Carlos Rafael Giani]

Hello, I have Marvell PJ4 based platform which didn't get extra GCC 
support until GCC 4.8. It added "marvellpj4" as an arch.
However, if for some reason GCC 4.7 is used, and --march=marvellpj4 is 
used, then building will fail.

Is there a way to add a --march=marvellpj4 flag to TUNE_CCARGS if and 
only if GCC >= 4.8 is used for cross compiling?

cheers