Implement support for Zig toolchain

[fwsGonzo]

Zig is can cross-compile to RISC-V on all relevant platforms. I've started work on this, but got stuck on C++ exceptions when (and only when) heap functions are overridden using native heap syscalls. Something in the __lcxx_override section gets evicted, possibly, or something stops working in libcxxabi. Either way a mystery.