`wasm2c` uses an interesting mechanism to "fake" the existence of cache
directories. However, `wasi_snapshot_preview1_fd_seek` was not correctly
integrated with this system, so previously crashed when run on a file in
a cache directory due to trying to call `fseek` on a `FILE *` which was
`NULL`.
I took a slightly unconventional approach to detecting endianness here. We have
no compiler/platform-specific preprocessor checks in the stage1 C code today,
and I think that's a property worth maintaining.
I moved part of the compiler that checks environment variables to the
standard library, so it doesn't have access to `build_options.only_c`
anymore, which means some environment variable checks are leaking into
the zig1.wasm build of zig. This logic still makes it return "no
environment variables found" though.
Multiple processes can sit waiting for the exclusive lock at the same
time, so we want to recheck whether it needs to be updated whenever
we get an exclusive lock.
This also fixes a race condition between one process truncating the
cache file and another process reading it without atomic locking.
