Cross-compiler prefix fixes

Allow overriding the prefix for the epiphany cross-compiler (eg, upstream gcc default is "epiphany-elf-").

Also remove automatic CROSS_PREFIX assignment (is it really needed? If someone is cross-compiling the host tools, I imagine they will set CROSS_PREFIX on their own).

The former is needed if one has built a cross-compiler from upstream gcc. The latter is just an annoyance from trying to test building this on my laptop that does not have a arm cross-compiler.