Does not support certain cross targets

• run-it is supposed to be built and run as a cross target executable. The codebase supports separating boot/test GHC, but doesn't seem to support passing a cross emulator to run target executables
• run-it uses dlopen which doesn't work on wasm. For target platforms without dlopen, we can't build run-it once and run it many times; instead, for each single test case, we need to build a specialized run-it that links with that test case's cmm code and outputs result directly.
• It's not immediately obvious to me whether things will work when host/target word size & endianness differ.

I'll self-assign this one since adding cross support looks like within my reach.