Fix shootout-ackermann

[abrown]

As reported in #238, the shootout-ackermann benchmark output did not match what was expected. While debugging this, several things became apparent:

• Relying on 0 as flags to open was the problem. wasi-libc defines O_RDONLY as 0x04000000, e.g., so it may be that passing 0 is problematic for some reason. Once O_RDONLY is used, the benchmark works as expected.
• assert() is not aborting the program as one might expect. There are a couple of definitions in wasi-libc (e.g., bottom-half and top-half) that seem to do the right thing but I did not observe an abort when I passed in bad paths to open().

Both of these issues seem wasi-libc related and might warrant some investigation there. This benchmark currently is building itself with wasi-sdk v19.0; by reverting this commit, here are the steps to replicate this issue:

$ benchmarks/build.sh benchmarks/shootout-ackermann/
$ RUST_LOG=trace cargo run -- benchmark --engine=engines/wasmtime/libengine.so --processes 1 --iterations-per-process 1 benchmarks/shootout-ackermann/benchmark.wasm

In any case, using O_RDONLY seems to fix the issue so we can close

238 and continue the discussion in the wasi-libc repository.

[jlb6740]

Great find Andrew!

[abrown]

After some discussion today, it became clear that at least the assert issue is of our own doing: at some point we "undefined" NDEBUG which changes how assert works in C generally. https://github.com/bytecodealliance/sightglass/pull/244 fixes this.