zig c++: fix libcxx build and add support for wasm32-wasi target.

[mathetake]

This is basically following wasi-sdk's Makefile, and notably I emulated the behavior of LIBCXX_ENABLE_FILESYSTEM:BOOL CMake flag, and completely disabled the threading APIs. This resolves #9044 without modifying libcxx. Ref: WebAssembly/wasi-sdk#125

[mathetake]

Now we can compile

#include <string>

int main(int argc, char **argv) {
    printf("test: %s", std::string("my-string").c_str());
    return 0;
}

but we got some strange imports inside the binary:

Import[9]:
 - func[0] sig=2 <__imported_wasi_snapshot_preview1_args_get> <- wasi_snapshot_preview1.args_get
 - func[1] sig=2 <__imported_wasi_snapshot_preview1_args_sizes_get> <- wasi_snapshot_preview1.args_sizes_get
 - func[2] sig=3 <__imported_wasi_snapshot_preview1_fd_close> <- wasi_snapshot_preview1.fd_close
 - func[3] sig=2 <__imported_wasi_snapshot_preview1_fd_fdstat_get> <- wasi_snapshot_preview1.fd_fdstat_get
 - func[4] sig=4 <__imported_wasi_snapshot_preview1_fd_seek> <- wasi_snapshot_preview1.fd_seek
 - func[5] sig=5 <__imported_wasi_snapshot_preview1_fd_write> <- wasi_snapshot_preview1.fd_write
 - func[6] sig=6 <__imported_wasi_snapshot_preview1_proc_exit> <- wasi_snapshot_preview1.proc_exit
 - func[7] sig=3 <std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::~basic_string()> <- env._ZNSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEED2Ev
 - func[8] sig=7 <std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >::__init(char const*, unsigned long)> <- env._ZNSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEE6__initEPKcm

will look into this now.

[mathetake]

OK looks like just libcxx isn't hooked up into linker well.

[mathetake]

Yeah finally it's started working! yay! @kubkon PTAL!

mathetake@mathetake ~/zig/build cpp-wasi
<< cat ~/zigcctest/test.cc
#include <cstdio>
#include <iostream>
#include <string>

int main() {
    std::string str = "hello wasi-sdk!";
    std::cout << str << std::endl;
    std::printf("%s\n", str.c_str());

    return 0;
}

mathetake@mathetake ~/zig/build cpp-wasi
>> /home/mathetake/zig-bootstrap/out/host/bin/zig c++ -target wasm32-wasi ~/zigcctest/test.cc

mathetake@mathetake ~/zig/build cpp-wasi 7s
>> wasmtime a.out       
hello wasi-sdk!
hello wasi-sdk!

[kubkon]

Excellent work! Thank you @mathetake!

[mathetake]

Thanks @kubkon for taking a look! 🥇 😄