[mktrxfw] avoid hard-coded triple of mips-gcc

[mizhka]

This change fixes build of mktrxfw on non-10.2 platforms. This is short-term bit, because of future plan to use Clang. For a while, let's extract path to mips-gcc from pkg. It's not nice, but it's less surprise thing.

[erikarn]

Shouldn't this just be a port that we require installed like a bunch of the other bits?

(I should do the same thing for all the other stuff in programs/ too, erk.)