autodetect toolchain prefix using priority lists

[wucke13]

Depending on the distro, arm gcc may have many names:

• arm-none-eabi-gcc
• arm-none-eabihf-gcc
• arm-linux-gnueabi
• arm-unknown-linux-gnueabi
• ... ?

We should be smart about which one we assume, instead of hardcoding one of them. It can then be passed to the cmake using -DCROSS_COMPILER_PREFIX=arm-none-eabihf- for example. As a last resort, using #1 the downstream consumer can override it.

[moritz-meier]

Update: sel4-sys should map its current cargo target to a list of acceptable, equivalent gcc cross-compilers, then look which of them is available and use that; otherwise fail with a helpful error message. sel4 and sel4-sys, sel4-rs and the rootserver application should in the end all be compiled to the same target or with related targets.