Proper support for cross-compiling

[mato]

Between #417 and #426 there's enough of a case for adding proper support for cross-compiling to the build system.

Ideally this should allow for all of:

1. Full cross-compilation of tender and bindings, at least to a different ARCH or toolchain on the same host. e.g. build for aarch64-linux-gnu on x86_64-linux-gnu.
2. Override toolchain used for tenders to allow for instrumentation (#417).
3. Other cases such as building the Genode bindings in-tree (#426).

[dinosaure]

The first point 1) "Full cross-compilation of tender and bindings" was done via #506 / #494. However, afl instrumentation is not implemented and I don't have any clues about the Genode support - but I remember that it was deleted via given PR.