Add at least a one-line comment on every
`#ifdef __wasilibc_unmodified_upstream` briefly indicating what its
purpose is.
While here, reenable some of the code that was previously disabled when
getenv wasn't supported, as getenv is now supported.
* Provide a public interface to preopened directory lookups.
For users of especially non-C compilers, provide an API for looking up
preopened directories. This is used internally in WASI libc to translate
from eg. `open` to `openat`, but it can now also be used by user code.
* Fix ssize_t redefinition mismatch.
Align definition with comment and previous define in __typedef_ssize_t.h
* Fix __typedef_ssize_t.h, redundancy in sys/types.h
This populates the repo with a very minimal set of files. This is just
enough to set out a basic outline. Hopefully as we move forward we'll
be replacing many of these parts, but this is just something basic to
get things started.
This tree isn't really usable yet, as it doesn't yet have an easy way
to obtain a compiler-rt/libgcc build.