stage2: Add limited WASI support for selfExePath and globalCacheDir

[topolarity]

Dependent on #11053. Resolves #10716.

This PR adds support for locating the Zig executable and the library and global cache directories, based on looking in the fixed "/zig" and "/cache" directories. It also bypasses the file-locking in "src/Cache.zig" and initializes the WASI preopens on startup.

The expected directories can be provided on the command-line using --mapdir, as follows:

wasmtime --dir=. --mapdir=/cache::"$HOME/.cache/zig" --mapdir=/zig::./stage2-wasm/ ./stage2-wasm/bin/zig.wasm

The reason for the fixed directories is that argv[0] on WASI is just the basename (any absolute/relative path information is deleted by the runtime) and there is no WASI-equivalent to procfs.

This is enough to get stage2 running without LLVM on WASI:

$ ./build/zig build -Dtarget=wasm32-wasi --zig-lib-dir lib --prefix $(pwd)/stage2-wasm -Dsingle-threaded -Drelease
$ alias zigwasm="wasmtime --dir=. --mapdir=/cache::\"$HOME/.cache/zig\" --mapdir=/zig::$(pwd)/stage2-wasm/ $(pwd)/stage2-wasm/bin/zig.wasm"
$ zigwasm build-exe ./simple_hello.zig -target x86_64-linux
$ chmod +x ./simple_hello
$ ./simple_hello
Hello, world!

This should be considered highly experimental - I haven't even checked the behavior tests or anything.

[Jarred-Sumner]

The other nice thing about WASI is it's not too hard to emulate a subset of it in browsers

[topolarity]

I've hit what seems to be an instability in the CI tests a couple times now, related to the test "POSIX file locking with fcntl".

Maybe the 1ms wait in that test is too low? Is this a known problem?

[matu3ba]

nit to reorder sentence and question on tests with recommended preopen

[topolarity]

Chose to make this dependent on #11053 - Let's try to get that one merged first

[Pyrolistical]

I tried making a build, but it seems to be broken now.

$ zig version
0.10.0-dev.3900+ab4b26d8a

$ cat .git/refs/heads/master 
ab4b26d8a6a36ccf75af8e25f0a1f7b88063b76f

$ zig build -Dtarget=wasm32-wasi --zig-lib-dir lib --prefix $(pwd)/stage2-wasm -Dsingle-threaded -Drelease
.../lib/std/os.zig:110:28: error: struct 'os.wasi.os.wasi' has no member named 'PATH_MAX'
pub const PATH_MAX = system.PATH_MAX;
                     ~~~~~~^~~~~~~~~
.../lib/std/os/wasi.zig:4:1: note: struct declared here
const std = @import("std");
^~~~~

[andrewrk]

I tried making a build, but it seems to be broken now.

#13229

Fixed in a4eb221