fix(wasip2): reactors to not export cli/run

[ricochet]

This change enables components to be built without a wasi:cli/run export.

• Command mode (-mexec-model=command or default): Links crt1-command.o which contains exports_wasi_cli_run_run
• Reactor mode (-mexec-model=reactor): Links crt1-reactor.o which only has _initialize

I have a bit of a chicken and egg problem after removing the run export from defined-symbols.txt which seems to be our only test for this. I was able to test by adding additional checks in src/llvm-project/clang/test/Driver/wasm-toolchain.c but this wasi-libc change needs to land first for those tests to pass.

cc @alexcrichton, there are a number of refactors in flight leading up to #700. No rush on this one, it's something that's bugged me for a while.

[alexcrichton]

Thanks! This looks reasonable to me. From what I can tell __main_void.c is included in libc.so and libc.a and it makes sense that this should live in crt1-command.o

[alexcrichton]

I think this TODO might still be applicable? Mind carrying it over?

[alexcrichton]

Hm, actually, on second though, this isn't actually used at all right now. If exports_wasi_cli_run_run is deleted entirely I believe everything should work. I'm investigating that separately, but @ricochet in the meantime if you want to just drop this symbol outright (which also seems good) I think that's reasonable

[alexcrichton]

Ok I tracked things down in #710, which purges the usage of the adapter on wasip2 targets. This PR is still useful in its own right, however, by deleting the symbol that nothing ended up using