Allow setting the target triple as an argument, eg --target-triple

If one's toolchain does not follow the expected pattern, for me it would be "riscv64-linux-gnu-", tuxmake does not appear to allow for setting the value of CROSS_COMPILE dynamically.

My CI carries around a bunch of toolchains, some of which set the vendor part of the triple to ISAABI & some of which are riscv{32,64}-unknown-gnu-. We've been trying to use tuxmake in a fork of NIPA for the linux-riscv mailing list's patchwork: https://github.com/ConchuOD/nipa/blob/reset/tests_rv/patch/build_boot_rv64/build_boot_rv64.sh

I've made tuxmake do what I want by cloning the repo in my pipeline, running sed to replace with the correct target triple in riscv.ini but this approach doesn't really scale - I can only do it once before starting NIPA and not on a per invocation of tuxmake level.

I had in mind actually implementing this before making an issue, but from a wee bit of hacking that I did do, CROSS_COMPILE has been "fenced off" so I decided not to implement anything in case there was an idealogical opposition to changing things there.